_.dropRightWhile(array, [predicate=_.identity])

9

_.dropRightWhile(array, [predicate=_.identity])

建立一个数组的切片，从结尾丢弃掉一些元素。被丢弃的元素是从结尾开始循环，直到predicate返回了false。predicate函数有三个参数，value，index，array。

参数

array (Array): 须要建立切片的数组
[predicate=_.identity] (Function): 遍历调用的方法

返回值

(Array): 返回一个新数组切片

例子

var users = [
  { 'user': 'barney',  'active': true },
  { 'user': 'fred',    'active': false },
  { 'user': 'pebbles', 'active': false }
];
 
_.dropRightWhile(users, function(o) { return !o.active; });
// => objects for ['barney']
 
// The `_.matches` iteratee shorthand.
_.dropRightWhile(users, { 'user': 'pebbles', 'active': false });
// => objects for ['barney', 'fred']
 
// The `_.matchesProperty` iteratee shorthand.
_.dropRightWhile(users, ['active', false]);
// => objects for ['barney']
 
// The `_.property` iteratee shorthand.
_.dropRightWhile(users, 'active');
// => objects for ['barney', 'fred', 'pebbles']

源代码：

/**
 * lodash (Custom Build) <https://lodash.com/>
 * Build: `lodash modularize exports="npm" -o ./`
 * Copyright jQuery Foundation and other contributors <https://jquery.org/>
 * Released under MIT license <https://lodash.com/license>
 * Based on Underscore.js 1.8.3 <http://underscorejs.org/LICENSE>
 * Copyright Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
 */

/** Used as the size to enable large array optimizations. */
var LARGE_ARRAY_SIZE = 200;

/** Used as the `TypeError` message for "Functions" methods. */
var FUNC_ERROR_TEXT = 'Expected a function';

/** Used to stand-in for `undefined` hash values. */
var HASH_UNDEFINED = '__lodash_hash_undefined__';

/** Used to compose bitmasks for comparison styles. */
var UNORDERED_COMPARE_FLAG = 1,
    PARTIAL_COMPARE_FLAG = 2;

/** Used as references for various `Number` constants. */
var INFINITY = 1 / 0,
    MAX_SAFE_INTEGER = 9007199254740991;

/** `Object#toString` result references. */
var argsTag = '[object Arguments]',
    arrayTag = '[object Array]',
    boolTag = '[object Boolean]',
    dateTag = '[object Date]',
    errorTag = '[object Error]',
    funcTag = '[object Function]',
    genTag = '[object GeneratorFunction]',
    mapTag = '[object Map]',
    numberTag = '[object Number]',
    objectTag = '[object Object]',
    promiseTag = '[object Promise]',
    regexpTag = '[object RegExp]',
    setTag = '[object Set]',
    stringTag = '[object String]',
    symbolTag = '[object Symbol]',
    weakMapTag = '[object WeakMap]';

var arrayBufferTag = '[object ArrayBuffer]',
    dataViewTag = '[object DataView]',
    float32Tag = '[object Float32Array]',
    float64Tag = '[object Float64Array]',
    int8Tag = '[object Int8Array]',
    int16Tag = '[object Int16Array]',
    int32Tag = '[object Int32Array]',
    uint8Tag = '[object Uint8Array]',
    uint8ClampedTag = '[object Uint8ClampedArray]',
    uint16Tag = '[object Uint16Array]',
    uint32Tag = '[object Uint32Array]';

/** Used to match property names within property paths. */
var reIsDeepProp = /\.|\[(?:[^[\]]*|(["'])(?:(?!\1)[^\\]|\\.)*?\1)\]/,
    reIsPlainProp = /^\w*$/,
    reLeadingDot = /^\./,
    rePropName = /[^.[\]]+|\[(?:(-?\d+(?:\.\d+)?)|(["'])((?:(?!\2)[^\\]|\\.)*?)\2)\]|(?=(?:\.|\[\])(?:\.|\[\]|$))/g;

/**
 * Used to match `RegExp`
 * [syntax characters](http://ecma-international.org/ecma-262/7.0/#sec-patterns).
 */
var reRegExpChar = /[\\^$.*+?()[\]{}|]/g;

/** Used to match backslashes in property paths. */
var reEscapeChar = /\\(\\)?/g;

/** Used to detect host constructors (Safari). */
var reIsHostCtor = /^\[object .+?Constructor\]$/;

/** Used to detect unsigned integer values. */
var reIsUint = /^(?:0|[1-9]\d*)$/;

/** Used to identify `toStringTag` values of typed arrays. */
var typedArrayTags = {};
typedArrayTags[float32Tag] = typedArrayTags[float64Tag] =
typedArrayTags[int8Tag] = typedArrayTags[int16Tag] =
typedArrayTags[int32Tag] = typedArrayTags[uint8Tag] =
typedArrayTags[uint8ClampedTag] = typedArrayTags[uint16Tag] =
typedArrayTags[uint32Tag] = true;
typedArrayTags[argsTag] = typedArrayTags[arrayTag] =
typedArrayTags[arrayBufferTag] = typedArrayTags[boolTag] =
typedArrayTags[dataViewTag] = typedArrayTags[dateTag] =
typedArrayTags[errorTag] = typedArrayTags[funcTag] =
typedArrayTags[mapTag] = typedArrayTags[numberTag] =
typedArrayTags[objectTag] = typedArrayTags[regexpTag] =
typedArrayTags[setTag] = typedArrayTags[stringTag] =
typedArrayTags[weakMapTag] = false;

/** Detect free variable `global` from Node.js. */
var freeGlobal = typeof global == 'object' && global && global.Object === Object && global;

/** Detect free variable `self`. */
var freeSelf = typeof self == 'object' && self && self.Object === Object && self;

/** Used as a reference to the global object. */
var root = freeGlobal || freeSelf || Function('return this')();

/** Detect free variable `exports`. */
var freeExports = typeof exports == 'object' && exports && !exports.nodeType && exports;

/** Detect free variable `module`. */
var freeModule = freeExports && typeof module == 'object' && module && !module.nodeType && module;

/** Detect the popular CommonJS extension `module.exports`. */
var moduleExports = freeModule && freeModule.exports === freeExports;

/** Detect free variable `process` from Node.js. */
var freeProcess = moduleExports && freeGlobal.process;

/** Used to access faster Node.js helpers. */
var nodeUtil = (function() {
  try {
    return freeProcess && freeProcess.binding('util');
  } catch (e) {}
}());

/* Node.js helper references. */
var nodeIsTypedArray = nodeUtil && nodeUtil.isTypedArray;

/**
 * A specialized version of `_.some` for arrays without support for iteratee
 * shorthands.
 *
 * @private
 * @param {Array} [array] The array to iterate over.
 * @param {Function} predicate The function invoked per iteration.
 * @returns {boolean} Returns `true` if any element passes the predicate check,
 *  else `false`.
 */
function arraySome(array, predicate) {
  var index = -1,
      length = array ? array.length : 0;

  while (++index < length) {
    if (predicate(array[index], index, array)) {
      return true;
    }
  }
  return false;
}

/**
 * The base implementation of `_.property` without support for deep paths.
 *
 * @private
 * @param {string} key The key of the property to get.
 * @returns {Function} Returns the new accessor function.
 */
//返回一个方法，这个方法返回object的对应key的对应value
function baseProperty(key) {
  return function(object) {
    return object == null ? undefined : object[key];
  };
}

/**
 * The base implementation of `_.times` without support for iteratee shorthands
 * or max array length checks.
 *
 * @private
 * @param {number} n The number of times to invoke `iteratee`.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {Array} Returns the array of results.
 */
//_times的基础实现，返回一个长度为n的数组，元素值是遍历器处理过的index
//n是循环调用遍历器的次数，iteratee是遍历器
function baseTimes(n, iteratee) {
  var index = -1,//循环索引
      result = Array(n);//结果数组

  while (++index < n) {
    result[index] = iteratee(index);
  }
  return result;
}

/**
 * The base implementation of `_.unary` without support for storing metadata.
 *
 * @private
 * @param {Function} func The function to cap arguments for.
 * @returns {Function} Returns the new capped function.
 */
//_.unary的基础实现，建立一个函数只接受一个参数，忽略以后全部参数
function baseUnary(func) {
  return function(value) {
    return func(value);
  };
}

/**
 * Gets the value at `key` of `object`.
 *
 * @private
 * @param {Object} [object] The object to query.
 * @param {string} key The key of the property to get.
 * @returns {*} Returns the property value.
 */
function getValue(object, key) {
  return object == null ? undefined : object[key];
}

/**
 * Checks if `value` is a host object in IE < 9.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a host object, else `false`.
 */
//检查一个值是不是宿主对象，在小于IE9的浏览器状况下，例如BOM和DOM对象
function isHostObject(value) {
  // Many host objects are `Object` objects that can coerce to strings
  // despite having improperly defined `toString` methods.
  //不少宿主对象能够强制转换成字符串尽管它们的toString方法定义并不正确
  var result = false;
  if (value != null && typeof value.toString != 'function') {
    try {
      result = !!(value + '');
    } catch (e) {}
  }
  return result;
}

/**
 * Converts `map` to its key-value pairs.
 *
 * @private
 * @param {Object} map The map to convert.
 * @returns {Array} Returns the key-value pairs.
 */
//将本身实现的兼容式map数据类型转变成key-value形式
function mapToArray(map) {
  var index = -1,//循环索引
      result = Array(map.size);//结果数组

  map.forEach(function(value, key) {//循环赋值
    /*
    {key1: value1, key2: value2, ...}
    变成：
    [[key1, value1], [key2, value2], ...]
    */
    result[++index] = [key, value];
  });
  return result;
}

/**
 * Creates a unary function that invokes `func` with its argument transformed.
 *
 * @private
 * @param {Function} func The function to wrap.
 * @param {Function} transform The argument transform.
 * @returns {Function} Returns the new function.
 */
function overArg(func, transform) {
  return function(arg) {
    return func(transform(arg));
  };
}

/**
 * Converts `set` to an array of its values.
 *
 * @private
 * @param {Object} set The set to convert.
 * @returns {Array} Returns the values.
 */
function setToArray(set) {
  var index = -1,
      result = Array(set.size);

  set.forEach(function(value) {
    result[++index] = value;
  });
  return result;
}

/** Used for built-in method references. */
var arrayProto = Array.prototype,
    funcProto = Function.prototype,
    objectProto = Object.prototype;

/** Used to detect overreaching core-js shims. */
var coreJsData = root['__core-js_shared__'];

/** Used to detect methods masquerading as native. */
var maskSrcKey = (function() {
  var uid = /[^.]+$/.exec(coreJsData && coreJsData.keys && coreJsData.keys.IE_PROTO || '');
  return uid ? ('Symbol(src)_1.' + uid) : '';
}());

/** Used to resolve the decompiled source of functions. */
var funcToString = funcProto.toString;

/** Used to check objects for own properties. */
var hasOwnProperty = objectProto.hasOwnProperty;

/**
 * Used to resolve the
 * [`toStringTag`](http://ecma-international.org/ecma-262/7.0/#sec-object.prototype.tostring)
 * of values.
 */
var objectToString = objectProto.toString;

/** Used to detect if a method is native. */
var reIsNative = RegExp('^' +
  funcToString.call(hasOwnProperty).replace(reRegExpChar, '\\$&')
  .replace(/hasOwnProperty|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?') + '$'
);

/** Built-in value references. */
var Symbol = root.Symbol,
    Uint8Array = root.Uint8Array,
    propertyIsEnumerable = objectProto.propertyIsEnumerable,
    splice = arrayProto.splice;

/* Built-in method references for those with the same name as other `lodash` methods. */
var nativeKeys = overArg(Object.keys, Object);

/* Built-in method references that are verified to be native. */
var DataView = getNative(root, 'DataView'),
    Map = getNative(root, 'Map'),
    Promise = getNative(root, 'Promise'),
    Set = getNative(root, 'Set'),
    WeakMap = getNative(root, 'WeakMap'),
    nativeCreate = getNative(Object, 'create');

/** Used to detect maps, sets, and weakmaps. */
var dataViewCtorString = toSource(DataView),
    mapCtorString = toSource(Map),
    promiseCtorString = toSource(Promise),
    setCtorString = toSource(Set),
    weakMapCtorString = toSource(WeakMap);

/** Used to convert symbols to primitives and strings. */
var symbolProto = Symbol ? Symbol.prototype : undefined,
    symbolValueOf = symbolProto ? symbolProto.valueOf : undefined,
    symbolToString = symbolProto ? symbolProto.toString : undefined;

/**
 * Creates a hash object.
 *
 * @private
 * @constructor
 * @param {Array} [entries] The key-value pairs to cache.
 */
function Hash(entries) {
  var index = -1,
      length = entries ? entries.length : 0;

  this.clear();
  while (++index < length) {
    var entry = entries[index];
    this.set(entry[0], entry[1]);
  }
}

/**
 * Removes all key-value entries from the hash.
 *
 * @private
 * @name clear
 * @memberOf Hash
 */
function hashClear() {
  this.__data__ = nativeCreate ? nativeCreate(null) : {};
}

/**
 * Removes `key` and its value from the hash.
 *
 * @private
 * @name delete
 * @memberOf Hash
 * @param {Object} hash The hash to modify.
 * @param {string} key The key of the value to remove.
 * @returns {boolean} Returns `true` if the entry was removed, else `false`.
 */
function hashDelete(key) {
  return this.has(key) && delete this.__data__[key];
}

/**
 * Gets the hash value for `key`.
 *
 * @private
 * @name get
 * @memberOf Hash
 * @param {string} key The key of the value to get.
 * @returns {*} Returns the entry value.
 */
function hashGet(key) {
  var data = this.__data__;
  if (nativeCreate) {
    var result = data[key];
    return result === HASH_UNDEFINED ? undefined : result;
  }
  return hasOwnProperty.call(data, key) ? data[key] : undefined;
}

/**
 * Checks if a hash value for `key` exists.
 *
 * @private
 * @name has
 * @memberOf Hash
 * @param {string} key The key of the entry to check.
 * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
 */
function hashHas(key) {
  var data = this.__data__;
  return nativeCreate ? data[key] !== undefined : hasOwnProperty.call(data, key);
}

/**
 * Sets the hash `key` to `value`.
 *
 * @private
 * @name set
 * @memberOf Hash
 * @param {string} key The key of the value to set.
 * @param {*} value The value to set.
 * @returns {Object} Returns the hash instance.
 */
function hashSet(key, value) {
  var data = this.__data__;
  data[key] = (nativeCreate && value === undefined) ? HASH_UNDEFINED : value;
  return this;
}

// Add methods to `Hash`.
Hash.prototype.clear = hashClear;
Hash.prototype['delete'] = hashDelete;
Hash.prototype.get = hashGet;
Hash.prototype.has = hashHas;
Hash.prototype.set = hashSet;

/**
 * Creates an list cache object.
 *
 * @private
 * @constructor
 * @param {Array} [entries] The key-value pairs to cache.
 */
function ListCache(entries) {
  var index = -1,
      length = entries ? entries.length : 0;

  this.clear();
  while (++index < length) {
    var entry = entries[index];
    this.set(entry[0], entry[1]);
  }
}

/**
 * Removes all key-value entries from the list cache.
 *
 * @private
 * @name clear
 * @memberOf ListCache
 */
function listCacheClear() {
  this.__data__ = [];
}

/**
 * Removes `key` and its value from the list cache.
 *
 * @private
 * @name delete
 * @memberOf ListCache
 * @param {string} key The key of the value to remove.
 * @returns {boolean} Returns `true` if the entry was removed, else `false`.
 */
function listCacheDelete(key) {
  var data = this.__data__,
      index = assocIndexOf(data, key);

  if (index < 0) {
    return false;
  }
  var lastIndex = data.length - 1;
  if (index == lastIndex) {
    data.pop();
  } else {
    splice.call(data, index, 1);
  }
  return true;
}

/**
 * Gets the list cache value for `key`.
 *
 * @private
 * @name get
 * @memberOf ListCache
 * @param {string} key The key of the value to get.
 * @returns {*} Returns the entry value.
 */
function listCacheGet(key) {
  var data = this.__data__,
      index = assocIndexOf(data, key);

  return index < 0 ? undefined : data[index][1];
}

/**
 * Checks if a list cache value for `key` exists.
 *
 * @private
 * @name has
 * @memberOf ListCache
 * @param {string} key The key of the entry to check.
 * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
 */
function listCacheHas(key) {
  return assocIndexOf(this.__data__, key) > -1;
}

/**
 * Sets the list cache `key` to `value`.
 *
 * @private
 * @name set
 * @memberOf ListCache
 * @param {string} key The key of the value to set.
 * @param {*} value The value to set.
 * @returns {Object} Returns the list cache instance.
 */
function listCacheSet(key, value) {
  var data = this.__data__,
      index = assocIndexOf(data, key);

  if (index < 0) {
    data.push([key, value]);
  } else {
    data[index][1] = value;
  }
  return this;
}

// Add methods to `ListCache`.
ListCache.prototype.clear = listCacheClear;
ListCache.prototype['delete'] = listCacheDelete;
ListCache.prototype.get = listCacheGet;
ListCache.prototype.has = listCacheHas;
ListCache.prototype.set = listCacheSet;

/**
 * Creates a map cache object to store key-value pairs.
 *
 * @private
 * @constructor
 * @param {Array} [entries] The key-value pairs to cache.
 */
function MapCache(entries) {
  var index = -1,
      length = entries ? entries.length : 0;

  this.clear();
  while (++index < length) {
    var entry = entries[index];
    this.set(entry[0], entry[1]);
  }
}

/**
 * Removes all key-value entries from the map.
 *
 * @private
 * @name clear
 * @memberOf MapCache
 */
function mapCacheClear() {
  this.__data__ = {
    'hash': new Hash,
    'map': new (Map || ListCache),
    'string': new Hash
  };
}

/**
 * Removes `key` and its value from the map.
 *
 * @private
 * @name delete
 * @memberOf MapCache
 * @param {string} key The key of the value to remove.
 * @returns {boolean} Returns `true` if the entry was removed, else `false`.
 */
function mapCacheDelete(key) {
  return getMapData(this, key)['delete'](key);
}

/**
 * Gets the map value for `key`.
 *
 * @private
 * @name get
 * @memberOf MapCache
 * @param {string} key The key of the value to get.
 * @returns {*} Returns the entry value.
 */
function mapCacheGet(key) {
  return getMapData(this, key).get(key);
}

/**
 * Checks if a map value for `key` exists.
 *
 * @private
 * @name has
 * @memberOf MapCache
 * @param {string} key The key of the entry to check.
 * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
 */
function mapCacheHas(key) {
  return getMapData(this, key).has(key);
}

/**
 * Sets the map `key` to `value`.
 *
 * @private
 * @name set
 * @memberOf MapCache
 * @param {string} key The key of the value to set.
 * @param {*} value The value to set.
 * @returns {Object} Returns the map cache instance.
 */
function mapCacheSet(key, value) {
  getMapData(this, key).set(key, value);
  return this;
}

// Add methods to `MapCache`.
MapCache.prototype.clear = mapCacheClear;
MapCache.prototype['delete'] = mapCacheDelete;
MapCache.prototype.get = mapCacheGet;
MapCache.prototype.has = mapCacheHas;
MapCache.prototype.set = mapCacheSet;

/**
 *
 * Creates an array cache object to store unique values.
 *
 * @private
 * @constructor
 * @param {Array} [values] The values to cache.
 */
function SetCache(values) {
  var index = -1,
      length = values ? values.length : 0;

  this.__data__ = new MapCache;
  while (++index < length) {
    this.add(values[index]);
  }
}

/**
 * Adds `value` to the array cache.
 *
 * @private
 * @name add
 * @memberOf SetCache
 * @alias push
 * @param {*} value The value to cache.
 * @returns {Object} Returns the cache instance.
 */
function setCacheAdd(value) {
  this.__data__.set(value, HASH_UNDEFINED);
  return this;
}

/**
 * Checks if `value` is in the array cache.
 *
 * @private
 * @name has
 * @memberOf SetCache
 * @param {*} value The value to search for.
 * @returns {number} Returns `true` if `value` is found, else `false`.
 */
function setCacheHas(value) {
  return this.__data__.has(value);
}

// Add methods to `SetCache`.
SetCache.prototype.add = SetCache.prototype.push = setCacheAdd;
SetCache.prototype.has = setCacheHas;

/**
 * Creates a stack cache object to store key-value pairs.
 *
 * @private
 * @constructor
 * @param {Array} [entries] The key-value pairs to cache.
 */
function Stack(entries) {
  this.__data__ = new ListCache(entries);
}

/**
 * Removes all key-value entries from the stack.
 *
 * @private
 * @name clear
 * @memberOf Stack
 */
function stackClear() {
  this.__data__ = new ListCache;
}

/**
 * Removes `key` and its value from the stack.
 *
 * @private
 * @name delete
 * @memberOf Stack
 * @param {string} key The key of the value to remove.
 * @returns {boolean} Returns `true` if the entry was removed, else `false`.
 */
function stackDelete(key) {
  return this.__data__['delete'](key);
}

/**
 * Gets the stack value for `key`.
 *
 * @private
 * @name get
 * @memberOf Stack
 * @param {string} key The key of the value to get.
 * @returns {*} Returns the entry value.
 */
function stackGet(key) {
  return this.__data__.get(key);
}

/**
 * Checks if a stack value for `key` exists.
 *
 * @private
 * @name has
 * @memberOf Stack
 * @param {string} key The key of the entry to check.
 * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
 */
function stackHas(key) {
  return this.__data__.has(key);
}

/**
 * Sets the stack `key` to `value`.
 *
 * @private
 * @name set
 * @memberOf Stack
 * @param {string} key The key of the value to set.
 * @param {*} value The value to set.
 * @returns {Object} Returns the stack cache instance.
 */
function stackSet(key, value) {
  var cache = this.__data__;
  if (cache instanceof ListCache) {
    var pairs = cache.__data__;
    if (!Map || (pairs.length < LARGE_ARRAY_SIZE - 1)) {
      pairs.push([key, value]);
      return this;
    }
    cache = this.__data__ = new MapCache(pairs);
  }
  cache.set(key, value);
  return this;
}

// Add methods to `Stack`.
Stack.prototype.clear = stackClear;
Stack.prototype['delete'] = stackDelete;
Stack.prototype.get = stackGet;
Stack.prototype.has = stackHas;
Stack.prototype.set = stackSet;

/**
 * Creates an array of the enumerable property names of the array-like `value`.
 *
 * @private
 * @param {*} value The value to query.
 * @param {boolean} inherited Specify returning inherited property names.
 * @returns {Array} Returns the array of property names.
 */
//建立一个包含一个array-like对象的全部自身可枚举属性的key的数组
function arrayLikeKeys(value, inherited) {
  // Safari 8.1 makes `arguments.callee` enumerable in strict mode.
  // Safari 9 makes `arguments.length` enumerable in strict mode.
  var result = (isArray(value) || isArguments(value))
    ? baseTimes(value.length, String)
    : [];
//结果数组，若是value是array或者arguments对象，调用baseTimes返回一个结果数组，元素是String化的索引值
  var length = result.length,//结果数组长度
      skipIndexes = !!length;

  for (var key in value) {//循环value对象
    if ((inherited || hasOwnProperty.call(value, key)) &&
        !(skipIndexes && (key == 'length' || isIndex(key, length)))) {
          //若是除了索引属性和length属性以外还有其余属性，加入结果数组
      result.push(key);
    }
  }
  return result;
}

/**
 * Gets the index at which the `key` is found in `array` of key-value pairs.
 *
 * @private
 * @param {Array} array The array to inspect.
 * @param {*} key The key to search for.
 * @returns {number} Returns the index of the matched value, else `-1`.
 */
function assocIndexOf(array, key) {
  var length = array.length;
  while (length--) {
    if (eq(array[length][0], key)) {
      return length;
    }
  }
  return -1;
}

/**
 * The base implementation of `_.get` without support for default values.
 *
 * @private
 * @param {Object} object The object to query.
 * @param {Array|string} path The path of the property to get.
 * @returns {*} Returns the resolved value.
 */
//根据path获取object上的属性值
//object须要获取属性值的对象
//path属性所在的路径
function baseGet(object, path) {
  path = isKey(path, object) ? [path] : castPath(path);
  //判断path是否是合法属性名，若是是就存成[path]，若是不是就调用castPath方法生成path的数组

  var index = 0,//循环索引
      length = path.length;//path数组的长度

  while (object != null && index < length) {//获取深层的值
    object = object[toKey(path[index++])];
  }
  return (index && index == length) ? object : undefined;//找到就返回，没找到返回undefined
}

/**
 * The base implementation of `getTag`.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
//调用Object.prototype.toString得到数据的类型tag
function baseGetTag(value) {
  return objectToString.call(value);
}

/**
 * The base implementation of `_.hasIn` without support for deep paths.
 *
 * @private
 * @param {Object} [object] The object to query.
 * @param {Array|string} key The key to check.
 * @returns {boolean} Returns `true` if `key` exists, else `false`.
 */
function baseHasIn(object, key) {
  return object != null && key in Object(object);
}

/**
 * The base implementation of `_.isEqual` which supports partial comparisons
 * and tracks traversed objects.
 *
 * @private
 * @param {*} value The value to compare.
 * @param {*} other The other value to compare.
 * @param {Function} [customizer] The function to customize comparisons.
 * @param {boolean} [bitmask] The bitmask of comparison flags.
 *  The bitmask may be composed of the following flags:
 *     1 - Unordered comparison
 *     2 - Partial comparison
 * @param {Object} [stack] Tracks traversed `value` and `other` objects.
 * @returns {boolean} Returns `true` if the values are equivalent, else `false`.
 */
//_.isEqual的基础实现，深度比较两个对象是不是相等的
//value要比较的对象，other要比较的另外一个对象，customizer自定义比较方法
//bitmask为1说明是无序的比较，为2说明是部分比较
//stack追踪传入的参数
function baseIsEqual(value, other, customizer, bitmask, stack) {
  if (value === other) {//若是value和other严格相等，直接返回true
    return true;
  }
  if (value == null || other == null || (!isObject(value) && !isObjectLike(other))) {
    //判断有值为空或者NaN，反正不是数组或对象的状况的状况
    return value !== value && other !== other;
  }
  return baseIsEqualDeep(value, other, baseIsEqual, customizer, bitmask, stack);
  //调用baseIsEqualDeep来判断
}

/**
 * A specialized version of `baseIsEqual` for arrays and objects which performs
 * deep comparisons and tracks traversed objects enabling objects with circular
 * references to be compared.
 *
 * @private
 * @param {Object} object The object to compare.
 * @param {Object} other The other object to compare.
 * @param {Function} equalFunc The function to determine equivalents of values.
 * @param {Function} [customizer] The function to customize comparisons.
 * @param {number} [bitmask] The bitmask of comparison flags. See `baseIsEqual`
 *  for more details.
 * @param {Object} [stack] Tracks traversed `object` and `other` objects.
 * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
 */
//深度比较数组和对象是否相等
function baseIsEqualDeep(object, other, equalFunc, customizer, bitmask, stack) {
  var objIsArr = isArray(object),//判断object是否是数组
      othIsArr = isArray(other),//判断other是否是数组
      objTag = arrayTag,//'[object Array]'
      othTag = arrayTag;//'[object Array]'

  if (!objIsArr) {
    objTag = getTag(object);
    objTag = objTag == argsTag ? objectTag : objTag;
  }
  if (!othIsArr) {
    othTag = getTag(other);//若是object不是数组，就获取object的数据类型tag
    othTag = othTag == argsTag ? objectTag : othTag;
  }
  var objIsObj = objTag == objectTag && !isHostObject(object),//object是对象类型且不是宿主对象
      othIsObj = othTag == objectTag && !isHostObject(other),//other是对象类型且不是宿主对象
      isSameTag = objTag == othTag;//object和other的类型tag同样

  if (isSameTag && !objIsObj) {//object和other的tag相等且它们不是本地对象类型
    stack || (stack = new Stack);//新建一个ListCache类型数据，ListCache其实就是一个本身实现的Map数据类型
    return (objIsArr || isTypedArray(object))
      ? equalArrays(object, other, equalFunc, customizer, bitmask, stack)
      : equalByTag(object, other, objTag, equalFunc, customizer, bitmask, stack);
      //若是都是数组类型或者typedArray类型就调用equalArrays方法判断
      //不然调用equalByTag判断
  }
  if (!(bitmask & PARTIAL_COMPARE_FLAG)) {//若是不是部分比较，且是object类型
    //此处__warpped__属性看不明白???
    var objIsWrapped = objIsObj && hasOwnProperty.call(object, '__wrapped__'),
        othIsWrapped = othIsObj && hasOwnProperty.call(other, '__wrapped__');

    if (objIsWrapped || othIsWrapped) {
      var objUnwrapped = objIsWrapped ? object.value() : object,
          othUnwrapped = othIsWrapped ? other.value() : other;

      stack || (stack = new Stack);
      return equalFunc(objUnwrapped, othUnwrapped, customizer, bitmask, stack);//递归调用baseIsEqual
    }
  }
  if (!isSameTag) {//若是toStringTag不同，直接返回false
    return false;
  }
  stack || (stack = new Stack);
  return equalObjects(object, other, equalFunc, customizer, bitmask, stack);
  //其余状况使用equalObjects来判断
}

/**
 * The base implementation of `_.isMatch` without support for iteratee shorthands.
 *
 * @private
 * @param {Object} object The object to inspect.
 * @param {Object} source The object of property values to match.
 * @param {Array} matchData The property names, values, and compare flags to match.
 * @param {Function} [customizer] The function to customize comparisons.
 * @returns {boolean} Returns `true` if `object` is a match, else `false`.
 */
//object，给定对象
//source，用来比较的源对象
//matchData，source参数的[key, value, boolean]的数组的形式，第三个布尔值代表当前值是否适合用===比较
//自定义比较函数
function baseIsMatch(object, source, matchData, customizer) {
  var index = matchData.length,//循环索引，从结尾开始循环
      length = index,//source长度
      noCustomizer = !customizer;//有没有传递自定义比较方法的标识

  if (object == null) {//若是object为空，若source有属性，返回false，若source无属性，返回true
    return !length;
  }
  object = Object(object);//强制转换object
  while (index--) {//按source属性长度循环
    var data = matchData[index];//当前matchData
    if ((noCustomizer && data[2])
          ? data[1] !== object[data[0]]
          : !(data[0] in object)
        ) {//若是没有自定义比较方法且当前source属性值适合===判断，也就是说是简单类型数据，就直接用!==判断是否不相等
          //不然用in来判断object是否不含有source key属性
      return false;
    }
  }
  //上面循环先循环一遍判断适合使用===直接判断的属性，下面循环再循环一遍判断其余须要深度循环判断的属性
  //上面循环结束后index循环索引变成0，如今再次正向循环
  while (++index < length) {
    data = matchData[index];//当前matchData
    var key = data[0],//当前source key
        objValue = object[key],//当前object 对应source key 的值
        srcValue = data[1];//当前source value

    if (noCustomizer && data[2]) {//若是没有自定义比较方法，且是简单类型，就判断objValue上是否不存在此属性
      //若是不存在直接返回false
      if (objValue === undefined && !(key in object)) {
        return false;
      }
    } else {
      var stack = new Stack;//新实例化一个Stack对象
      if (customizer) {//若是有自定义比较方法，就用自定义的比较
        var result = customizer(objValue, srcValue, key, object, source, stack);
      }
      if (!(result === undefined
            ? baseIsEqual(srcValue, objValue, customizer, UNORDERED_COMPARE_FLAG | PARTIAL_COMPARE_FLAG, stack)
            : result
          )) {//若是没有自定义比较方法，就用baseIsEqual来比较
        return false;
      }
    }
  }
  return true;
}

/**
 * The base implementation of `_.isNative` without bad shim checks.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a native function,
 *  else `false`.
 */
function baseIsNative(value) {
  if (!isObject(value) || isMasked(value)) {
    return false;
  }
  var pattern = (isFunction(value) || isHostObject(value)) ? reIsNative : reIsHostCtor;
  return pattern.test(toSource(value));
}

/**
 * The base implementation of `_.isTypedArray` without Node.js optimizations.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a typed array, else `false`.
 */
function baseIsTypedArray(value) {
  return isObjectLike(value) &&
    isLength(value.length) && !!typedArrayTags[objectToString.call(value)];
}

/**
 * The base implementation of `_.iteratee`.
 *
 * @private
 * @param {*} [value=_.identity] The value to convert to an iteratee.
 * @returns {Function} Returns the iteratee.
 */
//_.iteratee的基础实现，把一个值转换成一个迭代器函数
function baseIteratee(value) {
  // Don't store the `typeof` result in a variable to avoid a JIT bug in Safari 9.
  // See https://bugs.webkit.org/show_bug.cgi?id=156034 for more details.
  if (typeof value == 'function') {//若是value原本就是function，直接返回
    return value;
  }
  if (value == null) {//若是value是空，迭代器就使用事先定义好的identity方法
    //identity方法返回原值，不作操做
    return identity;
  }
  if (typeof value == 'object') {//若是value是对象
    return isArray(value)
      ? baseMatchesProperty(value[0], value[1])
      : baseMatches(value);
      //若是value是一个数组，调用baseMatchesProperty，则value[0]是path，获取对象上的属性的路径；value[1]是srcValue，和以后传入参数根据路径获取到的属性值比较用的值，baseMatchesProperty会返回一个方法，这个方法判断value[1]是否和传入的对象根据value[0]路径获取到的属性 是相等的，内部比较使用相似baseIsEqual的深层比较方法。相等返回true，不然false
      //若是value的类型不是一个数组，是一个对象，那么调用baseMatches，baseMatches建立一个方法，这个方法会使用部分的（partial）深度比较来比较给定的对象和source对象，若是给定对象拥有相等的属性值，就返回true,不然false。这里给baseMatches传入的value参数就是source，以后differenctBy循环的时候全部值都会和source比较
  }
  return property(value);
  //若是value都不是以上状况，那就调用property,建立一个方法，这个方法能够根据给定path属性路径返回给定对象的属性值
}

/**
 * The base implementation of `_.keys` which doesn't treat sparse arrays as dense.
 *
 * @private
 * @param {Object} object The object to query.
 * @returns {Array} Returns the array of property names.
 */
//_.keys的基础实现，将对象的可枚举属性的key组成一个数组返回
function baseKeys(object) {
  if (!isPrototype(object)) {//若是不是prototype对象，使用原生Object.keys方法
    return nativeKeys(object);
  }
  var result = [];//结果数组
  for (var key in Object(object)) {
    if (hasOwnProperty.call(object, key) && key != 'constructor') {
      //不等于constructor的自身属性push入结果数组
      result.push(key);
    }
  }
  return result;
}

/**
 * The base implementation of `_.matches` which doesn't clone `source`.
 *
 * @private
 * @param {Object} source The object of property values to match.
 * @returns {Function} Returns the new spec function.
 */
//_.matches方法的基础实现
//建立一个方法，这个方法会使用部分的（partial）深度比较来比较给定的对象和source对象，若是给定对象拥有相等的属性值，就返回true,不然false
function baseMatches(source) {
  var matchData = getMatchData(source);
  //getMatchData，把对象变成[key, value, boolean]的数组的形式，第三个布尔值代表当前值是否适合用===比较
  if (matchData.length == 1 && matchData[0][2]) {//若是source对象只有一个属性
    return matchesStrictComparable(matchData[0][0], matchData[0][1]);
    //matchesStrictComparable返回一个方法，这个方法用于比较传入对象object key值对应的值是否和给定值srcValue相等
    //matchData[0][0]就是key，matchData[0][1]就是srcValue
  }
  return function(object) {
    //若是source有多个属性，那么就调用baseIsMatch
    return object === source || baseIsMatch(object, source, matchData);
  };
}

/**
 * The base implementation of `_.matchesProperty` which doesn't clone `srcValue`.
 *
 * @private
 * @param {string} path The path of the property to get.
 * @param {*} srcValue The value to match.
 * @returns {Function} Returns the new spec function.
 */
//生成一个函数，这个函数能够深度比较对象的某个属性是否与给定值相等
//path参数是想要获取的属性的路径
//srcValue是用来匹配的值
function baseMatchesProperty(path, srcValue) {
  if (isKey(path) && isStrictComparable(srcValue)) {
    //若是path是属性名 而且 srcValue能够适用严格等于===来比较
    return matchesStrictComparable(toKey(path), srcValue);
    //返回一个返回object[key] === srcValue判断结果的函数
  }
  return function(object) {
    var objValue = get(object, path);//根据path获取object上对应健的值
    return (objValue === undefined && objValue === srcValue)
      ? hasIn(object, path)
      : baseIsEqual(srcValue, objValue, undefined, UNORDERED_COMPARE_FLAG | PARTIAL_COMPARE_FLAG);
      //返回布尔值，判断根据path获取到的值和给定值是否相等
      //若是获取到的值和用来比较的值都是undefined，那就object中是否存在这个path路径
      //hasIn判断path路径在给定object中是否存在
      //不然用baseIsEqual判断
  };
}

/**
 * A specialized version of `baseProperty` which supports deep paths.
 *
 * @private
 * @param {Array|string} path The path of the property to get.
 * @returns {Function} Returns the new accessor function.
 */
//返回一个方法，这个方法根据path返回object的对应属性值
function basePropertyDeep(path) {
  return function(object) {
    return baseGet(object, path);
  };
}

/**
 * The base implementation of `_.slice` without an iteratee call guard.
 *
 * @private
 * @param {Array} array The array to slice.
 * @param {number} [start=0] The start position.
 * @param {number} [end=array.length] The end position.
 * @returns {Array} Returns the slice of `array`.
 */
//建立一个数组array的切片，从起始索引到结束索引，不包括结束索引
function baseSlice(array, start, end) {
  var index = -1,//循环索引
      length = array.length;//数组长度

  if (start < 0) {//起始索引是负数处理，是负数就至关于从末尾往开头数，也就是和lengh相加
    start = -start > length ? 0 : (length + start);//和length相加后若是仍是小于0就等于0
  }
  end = end > length ? length : end;//结束索引若是大于length就让它等于length
  if (end < 0) {//处理结束索引是负数
    end += length;
  }
  length = start > end ? 0 : ((end - start) >>> 0);//根据start和end计算这个切片的长度，若是起始在结束后面那么切片长度为0，不然相减而且取整
  start >>>= 0;//start取整

  var result = Array(length);//结果切片数组
  while (++index < length) {//循环切片长度的次数，给结果数组每一项赋值
    result[index] = array[index + start];
  }
  return result;
}

/**
 * The base implementation of `_.toString` which doesn't convert nullish
 * values to empty strings.
 *
 * @private
 * @param {*} value The value to process.
 * @returns {string} Returns the string.
 */
//转换一个值为字符串，不会处理空值
function baseToString(value) {
  // Exit early for strings to avoid a performance hit in some environments.
  if (typeof value == 'string') {//若是已是字符串，直接返回
    return value;
  }
  if (isSymbol(value)) {//若是是Symbol对象，调用Symbol.prototype.toString处理
    return symbolToString ? symbolToString.call(value) : '';
  }
  var result = (value + '');//隐式转换
  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result;//处理-0的状况
}

/**
 * The base implementation of methods like `_.dropWhile` and `_.takeWhile`
 * without support for iteratee shorthands.
 *
 * @private
 * @param {Array} array The array to query.
 * @param {Function} predicate The function invoked per iteration.
 * @param {boolean} [isDrop] Specify dropping elements instead of taking them.
 * @param {boolean} [fromRight] Specify iterating from right to left.
 * @returns {Array} Returns the slice of `array`.
 */
//dropWhile的基础实现
//array须要处理的数组，predicate遍历器函数，isDrop指定丢弃或者获取元素，fromRight指定循环的方向
function baseWhile(array, predicate, isDrop, fromRight) {
  var length = array.length,//数组的长度
      index = fromRight ? length : -1;//循环索引，若是fromRight为true，从右往左循环，起始索引是数组长度；若是fromRight为false，从左往右循环，起始索引是-1

  while ((fromRight ? index-- : ++index < length) &&
    predicate(array[index], index, array)) {}
    //循环数组，length--从右往左循环，-1++从左往右循环
    //为数组循环值调用predicate，直到predicate返回false，循环索引index后面slice的时候有用
  return isDrop
    ? baseSlice(array, (fromRight ? 0 : index), (fromRight ? index + 1 : length))
    : baseSlice(array, (fromRight ? index + 1 : 0), (fromRight ? length : index));
    //isDrop为true的时候，若是从右往左丢弃，起始slice为0，结束slice为index+1；若是从左往右丢弃，起始slice为index，结束slice为length
    //isDrop为false的时候，若是从右往左获取，起始slice为index+1，结束slice为length；若是从左往右获取，起始slice为0，结束slice为index
}

/**
 * Casts `value` to a path array if it's not one.
 *
 * @private
 * @param {*} value The value to inspect.
 * @returns {Array} Returns the cast property path array.
 */
//把一个.分隔的path值转换成一个path数组
function castPath(value) {
  return isArray(value) ? value : stringToPath(value);
  //若是path值已是数组了，直接返回；若是不是调用stringToPath方法转换
}

/**
 * A specialized version of `baseIsEqualDeep` for arrays with support for
 * partial deep comparisons.
 *
 * @private
 * @param {Array} array The array to compare.
 * @param {Array} other The other array to compare.
 * @param {Function} equalFunc The function to determine equivalents of values.
 * @param {Function} customizer The function to customize comparisons.
 * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual`
 *  for more details.
 * @param {Object} stack Tracks traversed `array` and `other` objects.
 * @returns {boolean} Returns `true` if the arrays are equivalent, else `false`.
 */
//深度比较数组是否相等
//partical部分比较的意思就是，other的长度能够比array长，other在包含array全部元素的状况下，还能够有本身的独特于array的元素
function equalArrays(array, other, equalFunc, customizer, bitmask, stack) {
  //bitmask内部使用的时候值只能是1或2，1表明无序比较，2表明部分比较
  //bitmask按位与后，只有按位与操做符两边数字相等的状况下才会返回整数，不然返回0
  var isPartial = bitmask & PARTIAL_COMPARE_FLAG,//是否部分比较
      arrLength = array.length,//array长度
      othLength = other.length;//other长度

  if (arrLength != othLength && !(isPartial && othLength > arrLength)) {//长度不符合要求，直接返回false
    return false;
  }
  // Assume cyclic values are equal.
  //一开始先假设循环值都是相等的
  //stack默认是listCache对象，是本身实现的Map数据类型
  var stacked = stack.get(array);//获取stack上的array key对应的value
  if (stacked && stack.get(other)) {//若是stack key存在，就和other key比较
    return stacked == other;
  }
  var index = -1,//循环索引
      result = true,//一开始先假设循环值都是相等的
      //若是是无序比较，就实例化一个SetCache对象
      seen = (bitmask & UNORDERED_COMPARE_FLAG) ? new SetCache : undefined;

  stack.set(array, other);//给stack里存上array和other，交叉存
  stack.set(other, array);

  // Ignore non-index properties.
  while (++index < arrLength) {//循环array的长度
    var arrValue = array[index],//当前循环的array值
        othValue = other[index];//当前循环的other值

    if (customizer) {//若是提供了customizer比较方法，就用它来比较，partial部分比较要反过来传递array和other
      var compared = isPartial
        ? customizer(othValue, arrValue, index, other, array, stack)
        : customizer(arrValue, othValue, index, array, other, stack);
    }
    if (compared !== undefined) {//若是自定义比较有告终果，且为真，就continue继续下一次循环，不然result=false，跳出循环
      if (compared) {
        continue;
      }
      result = false;
      break;
    }
    // Recursively compare arrays (susceptible to call stack limits).
    //递归地比较数组元素值是否相等
    if (seen) {//若是建立了seen变量，说明是无序比较
      //seen是SetCache对象，也是利用key-value形式存储值
      //遍历other，若是有值和当前array的值相等，就存入seen中，而且arraySome返回true，不然返回false跳出循环
      if (!arraySome(other, function(othValue, othIndex) {
            if (!seen.has(othIndex) &&
                (arrValue === othValue || equalFunc(arrValue, othValue, customizer, bitmask, stack))) {
              return seen.add(othIndex);
            }
          })) {
        result = false;
        break;
      }
    } else if (!(
          arrValue === othValue ||
            equalFunc(arrValue, othValue, customizer, bitmask, stack)
        )) {//其余状况比较
      result = false;
      break;
    }
  }
  stack['delete'](array);//stack上删除array和other属性
  stack['delete'](other);
  return result;
}

/**
 * A specialized version of `baseIsEqualDeep` for comparing objects of
 * the same `toStringTag`.
 *
 * **Note:** This function only supports comparing values with tags of
 * `Boolean`, `Date`, `Error`, `Number`, `RegExp`, or `String`.
 *
 * @private
 * @param {Object} object The object to compare.
 * @param {Object} other The other object to compare.
 * @param {string} tag The `toStringTag` of the objects to compare.
 * @param {Function} equalFunc The function to determine equivalents of values.
 * @param {Function} customizer The function to customize comparisons.
 * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual`
 *  for more details.
 * @param {Object} stack Tracks traversed `object` and `other` objects.
 * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
 */
//比较toStringTag相等的两个对象是否相等
//object用来比较的对象
//other用来比较的另外一个对象
//tag比较对象的toStringTag
//equalFunc基础比较相等方法，用来递归调用
//customizer自定义比较方法
//bitmask是否无序比较或者部分比较的标志
//stack，key-value结构跟踪比较的对象
function equalByTag(object, other, tag, equalFunc, customizer, bitmask, stack) {
  switch (tag) {//根据toStringTag来比较object和other对象
    case dataViewTag:
    //'[object DataView]'
      if ((object.byteLength != other.byteLength) ||
          (object.byteOffset != other.byteOffset)) {
        return false;
      }
      object = object.buffer;
      other = other.buffer;

    case arrayBufferTag:
    //'[object ArrayBuffer]'
      if ((object.byteLength != other.byteLength) ||
          !equalFunc(new Uint8Array(object), new Uint8Array(other))) {
        return false;
      }
      return true;

    case boolTag:
    //'[object Boolean]'
    case dateTag:
    //'[object Date]'
    case numberTag:
    //'[object Number]'
      // Coerce booleans to `1` or `0` and dates to milliseconds.
      // Invalid dates are coerced to `NaN`.
      //强制转换布尔值到1或0，时间对象转换为毫秒，无效的时间转换为NaN
      return eq(+object, +other);

    case errorTag:
    //'[object Error]'
      return object.name == other.name && object.message == other.message;

    case regexpTag:
    //'[object RegExp]'
    case stringTag:
    //'[object String]'
      // Coerce regexes to strings and treat strings, primitives and objects,
      // as equal. See http://www.ecma-international.org/ecma-262/7.0/#sec-regexp.prototype.tostring
      // for more details.
      return object == (other + '');

    case mapTag:
    //'[object Map]'
      var convert = mapToArray;
      //将map转换成数组的方法
      /*
    {key1: value1, key2: value2, ...}
    变成：
    [[key1, value1], [key2, value2], ...]
    */
    case setTag:
    //'[object Set]'
      var isPartial = bitmask & PARTIAL_COMPARE_FLAG;//是否部分比较
      convert || (convert = setToArray);//将map或者set转换成数组的方法，mapToArray或者setToArray

      if (object.size != other.size && !isPartial) {//若是不是部分比较，且长度不同，直接返回false
        return false;
      }
      // Assume cyclic values are equal.
      //假设循环值都相等，从中判断不相等的状况
      var stacked = stack.get(object);
      if (stacked) {
        return stacked == other;
      }
      bitmask |= UNORDERED_COMPARE_FLAG;

      // Recursively compare objects (susceptible to call stack limits).
      stack.set(object, other);//在stack里把object和other都存上
      var result = equalArrays(convert(object), convert(other), equalFunc, customizer, bitmask, stack);
      //把object和other都转换成数组，而后用equalArrays方法来比较是否相等
      stack['delete'](object);//清除stack
      return result;

    case symbolTag:
    //'[object Symbol]'
      if (symbolValueOf) {//Symbol.prototype.valueOf
        return symbolValueOf.call(object) == symbolValueOf.call(other);
      }
  }
  return false;
}

/**
 * A specialized version of `baseIsEqualDeep` for objects with support for
 * partial deep comparisons.
 *
 * @private
 * @param {Object} object The object to compare.
 * @param {Object} other The other object to compare.
 * @param {Function} equalFunc The function to determine equivalents of values.
 * @param {Function} customizer The function to customize comparisons.
 * @param {number} bitmask The bitmask of comparison flags. See `baseIsEqual`
 *  for more details.
 * @param {Object} stack Tracks traversed `object` and `other` objects.
 * @returns {boolean} Returns `true` if the objects are equivalent, else `false`.
 */
//判断object类型深度比较是否相等，支持部分比较
function equalObjects(object, other, equalFunc, customizer, bitmask, stack) {
  var isPartial = bitmask & PARTIAL_COMPARE_FLAG,//是否部分比较
      objProps = keys(object),//Object.keys()，返回key组成的数组
      objLength = objProps.length,
      othProps = keys(other),//Object.keys()，返回key组成的数组
      othLength = othProps.length;

  if (objLength != othLength && !isPartial) {//若是不是部分比较，且object和other的key长度不同，返回false
    return false;
  }
  var index = objLength;//循环索引是object key数组的长度
  while (index--) {
    var key = objProps[index];//object 的key
    if (!(isPartial ? key in other : hasOwnProperty.call(other, key))) {
      //若是是部分比较，就用in来判断，不然用Object.prototype.hasOwnProperty
      //判断object的key是否other里也有
      //若是没有，返回false
      return false;
    }
  }
  // Assume cyclic values are equal.
  var stacked = stack.get(object);
  if (stacked && stack.get(other)) {
    return stacked == other;
  }
  var result = true;//开始循环以前假设循环的值都是相等的
  stack.set(object, other);
  stack.set(other, object);

  var skipCtor = isPartial;//部分比较，跳过constructor
  while (++index < objLength) {//循环
    key = objProps[index];//object的key
    var objValue = object[key],//object key对应的value
        othValue = other[key];//other key对应的value

    if (customizer) {//若是有自定义比较方法，就用自定义的比较
      var compared = isPartial
        ? customizer(othValue, objValue, key, other, object, stack)
        : customizer(objValue, othValue, key, object, other, stack);
    }
    // Recursively compare objects (susceptible to call stack limits).
    if (!(compared === undefined
          ? (objValue === othValue || equalFunc(objValue, othValue, customizer, bitmask, stack))
          : compared
        )) {
          //若是自定义比较失败就跳出循环，result=false
          //若是不是自定义比较，就用===比较，或者继续递归调用baseIsEqual来深度比较
      result = false;
      break;
    }
    skipCtor || (skipCtor = key == 'constructor');
  }
  if (result && !skipCtor) {//不是部分比较且object中有constructor属性，不跳过constructor属性，判断constructor是否同样
    var objCtor = object.constructor,
        othCtor = other.constructor;

    // Non `Object` object instances with different constructors are not equal.
    if (objCtor != othCtor &&
        ('constructor' in object && 'constructor' in other) &&
        !(typeof objCtor == 'function' && objCtor instanceof objCtor &&
          typeof othCtor == 'function' && othCtor instanceof othCtor)) {
      result = false;
    }
  }
  stack['delete'](object);
  stack['delete'](other);
  return result;
}

/**
 * Gets the data for `map`.
 *
 * @private
 * @param {Object} map The map to query.
 * @param {string} key The reference key.
 * @returns {*} Returns the map data.
 */
function getMapData(map, key) {
  var data = map.__data__;
  return isKeyable(key)
    ? data[typeof key == 'string' ? 'string' : 'hash']
    : data.map;
}

/**
 * Gets the property names, values, and compare flags of `object`.
 *
 * @private
 * @param {Object} object The object to query.
 * @returns {Array} Returns the match data of `object`.
 */
//获取对象中的属性名，属性值，和用来匹配的标记
function getMatchData(object) {
  var result = keys(object),//object的key组成的数组
      length = result.length;//key数组的长度

  while (length--) {//循环
    var key = result[length],//当前key
        value = object[key];//当前值

    result[length] = [key, value, isStrictComparable(value)];
    //重写result的当前值，变成[key, value, boolean]的形式，第三个布尔值代表当前值是否适合用===比较
  }
  return result;
}

/**
 * Gets the native function at `key` of `object`.
 *
 * @private
 * @param {Object} object The object to query.
 * @param {string} key The key of the method to get.
 * @returns {*} Returns the function if it's native, else `undefined`.
 */
function getNative(object, key) {
  var value = getValue(object, key);
  return baseIsNative(value) ? value : undefined;
}

/**
 * Gets the `toStringTag` of `value`.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
var getTag = baseGetTag;

// Fallback for data views, maps, sets, and weak maps in IE 11,
// for data views in Edge < 14, and promises in Node.js.
if ((DataView && getTag(new DataView(new ArrayBuffer(1))) != dataViewTag) ||
    (Map && getTag(new Map) != mapTag) ||
    (Promise && getTag(Promise.resolve()) != promiseTag) ||
    (Set && getTag(new Set) != setTag) ||
    (WeakMap && getTag(new WeakMap) != weakMapTag)) {
  getTag = function(value) {
    var result = objectToString.call(value),
        Ctor = result == objectTag ? value.constructor : undefined,
        ctorString = Ctor ? toSource(Ctor) : undefined;

    if (ctorString) {
      switch (ctorString) {
        case dataViewCtorString: return dataViewTag;
        case mapCtorString: return mapTag;
        case promiseCtorString: return promiseTag;
        case setCtorString: return setTag;
        case weakMapCtorString: return weakMapTag;
      }
    }
    return result;
  };
}

/**
 * Checks if `path` exists on `object`.
 *
 * @private
 * @param {Object} object The object to query.
 * @param {Array|string} path The path to check.
 * @param {Function} hasFunc The function to check properties.
 * @returns {boolean} Returns `true` if `path` exists, else `false`.
 */
//判断object中是否存在path路径，考虑深层path路径
//object给定对象，path给定路径，hasFunc只判断一层路径的基础方法
function hasPath(object, path, hasFunc) {
  path = isKey(path, object) ? [path] : castPath(path);//将路径转换成路径数组

  var result,//结果布尔值
      index = -1,//循环索引
      length = path.length;//路径数组的长度

  while (++index < length) {//循环路径数组
    var key = toKey(path[index]);//转换key值为合法key值
    if (!(result = object != null && hasFunc(object, key))) {
      //若是有哪一层不存在，就跳出循环且result赋值false
      break;
    }
    //不然继续获取下一层
    object = object[key];
  }
  if (result) {//若是循环结束后result为true，说明存在此路径，返回true
    return result;
  }
  var length = object ? object.length : 0;
  return !!length && isLength(length) && isIndex(key, length) &&
    (isArray(object) || isArguments(object));
  //判断object是数组的状况
  //有length属性，length属性是有效数字，判断最后一个key值是不是一个数组有效索引，object是数组或者是一个arguments对象
}

/**
 * Checks if `value` is a valid array-like index.
 *
 * @private
 * @param {*} value The value to check.
 * @param {number} [length=MAX_SAFE_INTEGER] The upper bounds of a valid index.
 * @returns {boolean} Returns `true` if `value` is a valid index, else `false`.
 */
//检查一个值是不是一个有效的array-like的索引
function isIndex(value, length) {
  length = length == null ? MAX_SAFE_INTEGER : length;//若是length是空就换成最大安全数字
  return !!length &&
    (typeof value == 'number' || reIsUint.test(value)) &&
    (value > -1 && value % 1 == 0 && value < length);
    //length存在
    //value是number值或者value符合reIsUint = /^(?:0|[1-9]\d*)$/正则
    //value是正数，value是整数，value小于length
}

/**
 * Checks if `value` is a property name and not a property path.
 *
 * @private
 * @param {*} value The value to check.
 * @param {Object} [object] The object to query keys on.
 * @returns {boolean} Returns `true` if `value` is a property name, else `false`.
 */
function isKey(value, object) {//value用于检查的值，object用于在其中查询是否存在value属性的对象
  if (isArray(value)) {//若是value是数组，返回false，说明value是一个属性路径
    return false;
  }
  var type = typeof value;
  if (type == 'number' || type == 'symbol' || type == 'boolean' ||
      value == null || isSymbol(value)) {//若是value是以上类型，则是属性名，返回true
    return true;
  }
  return reIsPlainProp.test(value) || !reIsDeepProp.test(value) ||
    (object != null && value in Object(object));
    //判断字符串的状况，第一个正则reIsPlainProp判断若是都是单词说明是属性名
    //第二个正则reIsDeepProp判断若是含有点.或者含有数组说明不是属性名
    //第三种状况若是value在object里能找到说明是属性名
}

/**
 * Checks if `value` is suitable for use as unique object key.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is suitable, else `false`.
 */
function isKeyable(value) {
  var type = typeof value;
  return (type == 'string' || type == 'number' || type == 'symbol' || type == 'boolean')
    ? (value !== '__proto__')
    : (value === null);
}

/**
 * Checks if `func` has its source masked.
 *
 * @private
 * @param {Function} func The function to check.
 * @returns {boolean} Returns `true` if `func` is masked, else `false`.
 */
function isMasked(func) {
  return !!maskSrcKey && (maskSrcKey in func);
}

/**
 * Checks if `value` is likely a prototype object.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a prototype, else `false`.
 */
function isPrototype(value) {
  var Ctor = value && value.constructor,
      proto = (typeof Ctor == 'function' && Ctor.prototype) || objectProto;

  return value === proto;
}

/**
 * Checks if `value` is suitable for strict equality comparisons, i.e. `===`.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` if suitable for strict
 *  equality comparisons, else `false`.
 */
//检查一个值用严格等于===来比较是否合适
//这个值不是NaN也不是对象就符合要求
function isStrictComparable(value) {
  return value === value && !isObject(value);
}

/**
 * A specialized version of `matchesProperty` for source values suitable
 * for strict equality comparisons, i.e. `===`.
 *
 * @private
 * @param {string} key The key of the property to get.
 * @param {*} srcValue The value to match.
 * @returns {Function} Returns the new spec function.
 */
//返回一个方法，这个方法用于比较传入对象object key值对应的值是否和给定值srcValue相等
function matchesStrictComparable(key, srcValue) {
  return function(object) {
    if (object == null) {//若是传入对象为空，返回false
      return false;
    }
    return object[key] === srcValue &&
      (srcValue !== undefined || (key in Object(object)));
      //给定值不为undefined且object中有此key值，且key对应值和给定值严格严格相等，就返回true
  };
}

/**
 * Converts `string` to a property path array.
 *
 * @private
 * @param {string} string The string to convert.
 * @returns {Array} Returns the property path array.
 */
//转换字符串值变为属性path数组
var stringToPath = memoize(function(string) {
  string = toString(string);//将值转换成字符串

  var result = [];//结果数组
  if (reLeadingDot.test(string)) {
    //reLeadingDot = /^\./，判断字符串的开头若是是一个点，结果数组第一个就push一个空字符串
    result.push('');
  }
  string.replace(rePropName, function(match, number, quote, string) {
    //rePropName = /[^.[\]]+|\[(?:(-?\d+(?:\.\d+)?)|(["'])((?:(?!\2)[^\\]|\\.)*?)\2)\]|(?=(?:\.|\[\])(?:\.|\[\]|$))/g;
    //rePropName这个正则匹配三种形式
    //?:是非捕获分组
    //第一种形式：不是点.也不是[或者]的任意字符组成的字符串，例如：'aaassssdfdffdf'
    //第二种形式：判断取数组下标的状况，例如：a[0].b.c.a[0][1]
    //第三种形式：？？？
    result.push(quote ? string.replace(reEscapeChar, '$1') : (number || match));
    //reEscapeChar = /\\(\\)?/g
    //match是匹配到的内容，number是第二种形式的第一个捕获分组，quote是第二种形式的第二个捕获分组
    //有quote说明是带引号的内容，再执行一次replace操做，？？？
  });
  return result;
});

/**
 * Converts `value` to a string key if it's not a string or symbol.
 *
 * @private
 * @param {*} value The value to inspect.
 * @returns {string|symbol} Returns the key.
 */
//若是一个值不是string或者symbol，就把它转换成string
function toKey(value) {
  if (typeof value == 'string' || isSymbol(value)) {//若是这个值是字符串或者symbol，直接返回
    return value;
  }
  var result = (value + '');//转换成字符串
  return (result == '0' && (1 / value) == -INFINITY) ? '-0' : result;//判断value是负0的状况
}

/**
 * Converts `func` to its source code.
 *
 * @private
 * @param {Function} func The function to process.
 * @returns {string} Returns the source code.
 */
function toSource(func) {
  if (func != null) {
    try {
      return funcToString.call(func);
    } catch (e) {}
    try {
      return (func + '');
    } catch (e) {}
  }
  return '';
}

/**
 * Creates a slice of `array` excluding elements dropped from the end.
 * Elements are dropped until `predicate` returns falsey. The predicate is
 * invoked with three arguments: (value, index, array).
 *
 * @static
 * @memberOf _
 * @since 3.0.0
 * @category Array
 * @param {Array} array The array to query.
 * @param {Function} [predicate=_.identity] The function invoked per iteration.
 * @returns {Array} Returns the slice of `array`.
 * @example
 *
 * var users = [
 *   { 'user': 'barney',  'active': true },
 *   { 'user': 'fred',    'active': false },
 *   { 'user': 'pebbles', 'active': false }
 * ];
 *
 * _.dropRightWhile(users, function(o) { return !o.active; });
 * // => objects for ['barney']
 *
 * // The `_.matches` iteratee shorthand.
 * _.dropRightWhile(users, { 'user': 'pebbles', 'active': false });
 * // => objects for ['barney', 'fred']
 *
 * // The `_.matchesProperty` iteratee shorthand.
 * _.dropRightWhile(users, ['active', false]);
 * // => objects for ['barney']
 *
 * // The `_.property` iteratee shorthand.
 * _.dropRightWhile(users, 'active');
 * // => objects for ['barney', 'fred', 'pebbles']
 */
//建立一个数组的切片，从结尾开始排除掉一些元素，直到根据predicate参数返回false的时候的index就是slice的结束索引
function dropRightWhile(array, predicate) {
  return (array && array.length)
    ? baseWhile(array, baseIteratee(predicate, 3), true, true)
    : [];
    //若是数组不为空且长度不为0，就调用baseWhile来处理，不然返回空数组
    //baseWhile第一个参数array为处理的数组，第二个参数调用baseIteratee生成一个为判断须要丢弃的元素的函数，第三个参数isDrop丢弃仍是获取元素，第四个参数FromRight是否从右往左判断要丢弃的元素
}

/**
 * Creates a function that memoizes the result of `func`. If `resolver` is
 * provided, it determines the cache key for storing the result based on the
 * arguments provided to the memoized function. By default, the first argument
 * provided to the memoized function is used as the map cache key. The `func`
 * is invoked with the `this` binding of the memoized function.
 *
 * **Note:** The cache is exposed as the `cache` property on the memoized
 * function. Its creation may be customized by replacing the `_.memoize.Cache`
 * constructor with one whose instances implement the
 * [`Map`](http://ecma-international.org/ecma-262/7.0/#sec-properties-of-the-map-prototype-object)
 * method interface of `delete`, `get`, `has`, and `set`.
 *
 * @static
 * @memberOf _
 * @since 0.1.0
 * @category Function
 * @param {Function} func The function to have its output memoized.
 * @param {Function} [resolver] The function to resolve the cache key.
 * @returns {Function} Returns the new memoized function.
 * @example
 *
 * var object = { 'a': 1, 'b': 2 };
 * var other = { 'c': 3, 'd': 4 };
 *
 * var values = _.memoize(_.values);
 * values(object);
 * // => [1, 2]
 *
 * values(other);
 * // => [3, 4]
 *
 * object.a = 2;
 * values(object);
 * // => [1, 2]
 *
 * // Modify the result cache.
 * values.cache.set(object, ['a', 'b']);
 * values(object);
 * // => ['a', 'b']
 *
 * // Replace `_.memoize.Cache`.
 * _.memoize.Cache = WeakMap;
 */
//建立一个能够缓存func的结果的函数，resolver用来解析缓存result的键
function memoize(func, resolver) {
  if (typeof func != 'function' || (resolver && typeof resolver != 'function')) {
    //func和resolver（若是提供的话）若是不是函数，抛错误
    throw new TypeError(FUNC_ERROR_TEXT);
  }
  var memoized = function() {
    var args = arguments,
        key = resolver ? resolver.apply(this, args) : args[0],
        //若是提供了resolver，就用resolver来肯定key，不然key就是memoized的第一个参数
        cache = memoized.cache;//cashe对象

    if (cache.has(key)) {//若是cache中已经有此key，就直接获取后返回
      return cache.get(key);
    }
    var result = func.apply(this, args);//不然调用func计算出key对应的value
    memoized.cache = cache.set(key, result);//将新value存入cache后返回
    return result;
  };
  memoized.cache = new (memoize.Cache || MapCache);//cache使用自定义的MapCache类型
  return memoized;
}

// Assign cache to `_.memoize`.
memoize.Cache = MapCache;

/**
 * Performs a
 * [`SameValueZero`](http://ecma-international.org/ecma-262/7.0/#sec-samevaluezero)
 * comparison between two values to determine if they are equivalent.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to compare.
 * @param {*} other The other value to compare.
 * @returns {boolean} Returns `true` if the values are equivalent, else `false`.
 * @example
 *
 * var object = { 'a': 1 };
 * var other = { 'a': 1 };
 *
 * _.eq(object, object);
 * // => true
 *
 * _.eq(object, other);
 * // => false
 *
 * _.eq('a', 'a');
 * // => true
 *
 * _.eq('a', Object('a'));
 * // => false
 *
 * _.eq(NaN, NaN);
 * // => true
 */
//判断两个值是否相等
function eq(value, other) {
  return value === other || (value !== value && other !== other);
}

/**
 * Checks if `value` is likely an `arguments` object.
 *
 * @static
 * @memberOf _
 * @since 0.1.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is an `arguments` object,
 *  else `false`.
 * @example
 *
 * _.isArguments(function() { return arguments; }());
 * // => true
 *
 * _.isArguments([1, 2, 3]);
 * // => false
 */
//判断一个值是一个相似arguments对象 
function isArguments(value) {
  // Safari 8.1 makes `arguments.callee` enumerable in strict mode.
  return isArrayLikeObject(value) && hasOwnProperty.call(value, 'callee') &&
    (!propertyIsEnumerable.call(value, 'callee') || objectToString.call(value) == argsTag);
    //value是一个array-like对象，value有名为callee的自身属性，callee属性不可枚举，对象的toString标签是'[object Arguments]'
}

/**
 * Checks if `value` is classified as an `Array` object.
 *
 * @static
 * @memberOf _
 * @since 0.1.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is an array, else `false`.
 * @example
 *
 * _.isArray([1, 2, 3]);
 * // => true
 *
 * _.isArray(document.body.children);
 * // => false
 *
 * _.isArray('abc');
 * // => false
 *
 * _.isArray(_.noop);
 * // => false
 */
//判断一个值是否是数组对象
var isArray = Array.isArray;

/**
 * Checks if `value` is array-like. A value is considered array-like if it's
 * not a function and has a `value.length` that's an integer greater than or
 * equal to `0` and less than or equal to `Number.MAX_SAFE_INTEGER`.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is array-like, else `false`.
 * @example
 *
 * _.isArrayLike([1, 2, 3]);
 * // => true
 *
 * _.isArrayLike(document.body.children);
 * // => true
 *
 * _.isArrayLike('abc');
 * // => true
 *
 * _.isArrayLike(_.noop);
 * // => false
 */
//判断一个值是不是一个array-like
//规则：不等于null，不是function类型，而且有length属性，length是大于0小于Number.MAX_SAFE_INTEGER的整数
function isArrayLike(value) {
  return value != null && isLength(value.length) && !isFunction(value);
}

/**
 * This method is like `_.isArrayLike` except that it also checks if `value`
 * is an object.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is an array-like object,
 *  else `false`.
 * @example
 *
 * _.isArrayLikeObject([1, 2, 3]);
 * // => true
 *
 * _.isArrayLikeObject(document.body.children);
 * // => true
 *
 * _.isArrayLikeObject('abc');
 * // => false
 *
 * _.isArrayLikeObject(_.noop);
 * // => false
 */
//判断一个值是否是一个array-like对象
//isObjectLike判断一个值是不是一个object-like，规则是：typeof返回object，而且不是null
//isArrayLike判断一个值是不是一个array-like，规则：不等于null，不是function类型，而且有length属性，length是大于0小于Number.MAX_SAFE_INTEGER的整数
function isArrayLikeObject(value) {
  return isObjectLike(value) && isArrayLike(value);
}

/**
 * Checks if `value` is classified as a `Function` object.
 *
 * @static
 * @memberOf _
 * @since 0.1.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a function, else `false`.
 * @example
 *
 * _.isFunction(_);
 * // => true
 *
 * _.isFunction(/abc/);
 * // => false
 */
//判断一个值是不是Function对象
function isFunction(value) {
  // The use of `Object#toString` avoids issues with the `typeof` operator
  // in Safari 8-9 which returns 'object' for typed array and other constructors.
  var tag = isObject(value) ? objectToString.call(value) : '';
  //获取value的toString tag
  return tag == funcTag || tag == genTag;
  //若是toString tag等于[object Function]，或者[object GeneratorFunction]，说明是function
}

/**
 * Checks if `value` is a valid array-like length.
 *
 * **Note:** This method is loosely based on
 * [`ToLength`](http://ecma-international.org/ecma-262/7.0/#sec-tolength).
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a valid length, else `false`.
 * @example
 *
 * _.isLength(3);
 * // => true
 *
 * _.isLength(Number.MIN_VALUE);
 * // => false
 *
 * _.isLength(Infinity);
 * // => false
 *
 * _.isLength('3');
 * // => false
 */
//判断一个值是不是一个有效的array-like对象的length属性
function isLength(value) {
  return typeof value == 'number' &&
    value > -1 && value % 1 == 0 && value <= MAX_SAFE_INTEGER;
    //是number类型，是正数，是整数不是小数，在最大数字范围内
}

/**
 * Checks if `value` is the
 * [language type](http://www.ecma-international.org/ecma-262/7.0/#sec-ecmascript-language-types)
 * of `Object`. (e.g. arrays, functions, objects, regexes, `new Number(0)`, and `new String('')`)
 *
 * @static
 * @memberOf _
 * @since 0.1.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is an object, else `false`.
 * @example
 *
 * _.isObject({});
 * // => true
 *
 * _.isObject([1, 2, 3]);
 * // => true
 *
 * _.isObject(_.noop);
 * // => true
 *
 * _.isObject(null);
 * // => false
 */
//判断一个值是不是一个对象
function isObject(value) {
  var type = typeof value;
  return !!value && (type == 'object' || type == 'function');
}

/**
 * Checks if `value` is object-like. A value is object-like if it's not `null`
 * and has a `typeof` result of "object".
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is object-like, else `false`.
 * @example
 *
 * _.isObjectLike({});
 * // => true
 *
 * _.isObjectLike([1, 2, 3]);
 * // => true
 *
 * _.isObjectLike(_.noop);
 * // => false
 *
 * _.isObjectLike(null);
 * // => false
 */
//判断一个值是不是一个object-like，规则是：typeof返回object，而且不是null
function isObjectLike(value) {
  return !!value && typeof value == 'object';
}

/**
 * Checks if `value` is classified as a `Symbol` primitive or object.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a symbol, else `false`.
 * @example
 *
 * _.isSymbol(Symbol.iterator);
 * // => true
 *
 * _.isSymbol('abc');
 * // => false
 */
//判断一个值是不是原生的Symbol对象
function isSymbol(value) {
  return typeof value == 'symbol' ||
    (isObjectLike(value) && objectToString.call(value) == symbolTag);
}

/**
 * Checks if `value` is classified as a typed array.
 *
 * @static
 * @memberOf _
 * @since 3.0.0
 * @category Lang
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is a typed array, else `false`.
 * @example
 *
 * _.isTypedArray(new Uint8Array);
 * // => true
 *
 * _.isTypedArray([]);
 * // => false
 */
//判断一个值是typedArray类型，若是是nodejs环境就使用nodejs自带的优化方法
var isTypedArray = nodeIsTypedArray ? baseUnary(nodeIsTypedArray) : baseIsTypedArray;

/**
 * Converts `value` to a string. An empty string is returned for `null`
 * and `undefined` values. The sign of `-0` is preserved.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Lang
 * @param {*} value The value to process.
 * @returns {string} Returns the string.
 * @example
 *
 * _.toString(null);
 * // => ''
 *
 * _.toString(-0);
 * // => '-0'
 *
 * _.toString([1, 2, 3]);
 * // => '1,2,3'
 */
//将一个值转换成字符串值，空值会返回空字符串
function toString(value) {
  return value == null ? '' : baseToString(value);
}

/**
 * Gets the value at `path` of `object`. If the resolved value is
 * `undefined`, the `defaultValue` is returned in its place.
 *
 * @static
 * @memberOf _
 * @since 3.7.0
 * @category Object
 * @param {Object} object The object to query.
 * @param {Array|string} path The path of the property to get.
 * @param {*} [defaultValue] The value returned for `undefined` resolved values.
 * @returns {*} Returns the resolved value.
 * @example
 *
 * var object = { 'a': [{ 'b': { 'c': 3 } }] };
 *
 * _.get(object, 'a[0].b.c');
 * // => 3
 *
 * _.get(object, ['a', '0', 'b', 'c']);
 * // => 3
 *
 * _.get(object, 'a.b.c', 'default');
 * // => 'default'
 */
//根据path来获取object上对应健的值
function get(object, path, defaultValue) {
  var result = object == null ? undefined : baseGet(object, path);//根据路径获取值
  return result === undefined ? defaultValue : result;//若是结果是undefined，返回默认值
}

/**
 * Checks if `path` is a direct or inherited property of `object`.
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Object
 * @param {Object} object The object to query.
 * @param {Array|string} path The path to check.
 * @returns {boolean} Returns `true` if `path` exists, else `false`.
 * @example
 *
 * var object = _.create({ 'a': _.create({ 'b': 2 }) });
 *
 * _.hasIn(object, 'a');
 * // => true
 *
 * _.hasIn(object, 'a.b');
 * // => true
 *
 * _.hasIn(object, ['a', 'b']);
 * // => true
 *
 * _.hasIn(object, 'b');
 * // => false
 */
//判断path路径在给定object中是否存在
function hasIn(object, path) {
  return object != null && hasPath(object, path, baseHasIn);
}

/**
 * Creates an array of the own enumerable property names of `object`.
 *
 * **Note:** Non-object values are coerced to objects. See the
 * [ES spec](http://ecma-international.org/ecma-262/7.0/#sec-object.keys)
 * for more details.
 *
 * @static
 * @since 0.1.0
 * @memberOf _
 * @category Object
 * @param {Object} object The object to query.
 * @returns {Array} Returns the array of property names.
 * @example
 *
 * function Foo() {
 *   this.a = 1;
 *   this.b = 2;
 * }
 *
 * Foo.prototype.c = 3;
 *
 * _.keys(new Foo);
 * // => ['a', 'b'] (iteration order is not guaranteed)
 *
 * _.keys('hi');
 * // => ['0', '1']
 */
//将对象可枚举的属性的key提取出来建立一个新数组，与Object.keys同样的效果
function keys(object) {
  return isArrayLike(object) ? arrayLikeKeys(object) : baseKeys(object);
  //先判断object是不是array-like对象，若是是调用arrayLikeKeys，若是不是调用baseKeys
}

/**
 * This method returns the first argument it receives.
 *
 * @static
 * @since 0.1.0
 * @memberOf _
 * @category Util
 * @param {*} value Any value.
 * @returns {*} Returns `value`.
 * @example
 *
 * var object = { 'a': 1 };
 *
 * console.log(_.identity(object) === object);
 * // => true
 */
//用做默认迭代器，直接返回接收到的第一个参数
function identity(value) {
  return value;
}

/**
 * Creates a function that returns the value at `path` of a given object.
 *
 * @static
 * @memberOf _
 * @since 2.4.0
 * @category Util
 * @param {Array|string} path The path of the property to get.
 * @returns {Function} Returns the new accessor function.
 * @example
 *
 * var objects = [
 *   { 'a': { 'b': 2 } },
 *   { 'a': { 'b': 1 } }
 * ];
 *
 * _.map(objects, _.property('a.b'));
 * // => [2, 1]
 *
 * _.map(_.sortBy(objects, _.property(['a', 'b'])), 'a.b');
 * // => [1, 2]
 */
//建立一个方法，这个方法能够根据给定path属性路径返回给定对象的属性值
function property(path) {
  return isKey(path) ? baseProperty(toKey(path)) : basePropertyDeep(path);
  //先判断path是key仍是path
  //若是是key，调用baseProperty，返回一个方法，这个方法返回object的对应key的对应value
  //若是是路径，调用basePropertyDeep，返回一个方法，这个方法根据path返回object的对应属性值
}

module.exports = dropRightWhile;