平时使用的字符串应该是primitive类型,应该是not an object and has no methodsjavascript
const str = 'hello';
console.log(str.charAt(0));
// output: h
Object.prototype.toString.call(str)
// output: [object String]复制代码
N次循环技巧java
for(let i = 0; i < 5; i++) {
// ...
}
Array.apply(null, Array(5)).forEach(() => {
// ...
});
_.times(5, () => {
// ...
};复制代码
深层次查找属性git
const ownerArr = [{
"owner": "Colin",
"pets": [{"name":"dog1"}, {"name": "dog2"}]
}, {
"owner": "John",
"pets": [{"name":"dog3"}, {"name": "dog4"}]
}];
ownerArr.map(owner => {
return owner.pets[0].name;
});
_.map(ownerArr, 'pets[0].name');复制代码
数组独立es6
Array.apply(null, Array(6)).map( (item, index) => {
return "ball_" + index;
});
_.times(6, _.uniqueId.bind(null, 'ball_'));
_.times(6, _.partial(_.uniqueId, 'ball_'));
// output: [ball_0, ball_1, ball_2, ball_3, ball_4, ball_5]复制代码
对象扩展(能够直接用Object.assgin(), 底层同样的实现)github
Object.prototype.extend = obj => {
for (let i in obj) {
if (obj.hasOwnProperty(i)) {
this[i] = obj[i];
}
}
};
const objA = {"name": "colin", "car": "suzuki"};
const objB = {"name": "james", "age": 17};
objA.extend(objB);
console.log(objA);
// {"name": "james", "age": 17, "car": "suzuki"};
_.assign(objA, objB);
// {"name": "james", "age": 17, "car": "suzuki"};
// ES6
Objetct.assign({}, objA, objB);
// {"name": "james", "age": 17, "car": "suzuki"};
//_.assign 是浅拷贝,因此会覆盖name复制代码
补充做用域:数组
const test = '1';
testOne() {
return testTwo{
cosole.log(test);
};
const test = '2';
}
testOne()();
// output: undefined
const test = '1';
testOne() {
return testTwo{
console.log(test);
};
test = '2';
}
// output: 1;复制代码
由于从新定义了const,他在搜索做用域时候,会自上到下搜索声明的变量,若是没有声明,查找才会进去下一层,此处输出undefined,由于在testOne()里面const以前就使用了test,因此就输出了undefined,而在第二个例子里面没有声明test,因此他就跳转出去,去下一层寻找test,即输出为1bash
const a = 1;
b(){
const a = b = 2;
}
console.log(a, b);
// 抛出异常,由于b没有定义
b();
console.log(a, b);
//output: 1,2;
// const a = b = 2 等价于 在全局声明const b = 2; 内部声明const a = b;由于=运算符是重右像左运算的复制代码