JavaScript数组循环的几种写法，包含ES6

利用Javascript map(),reduce()和filter()数组方法能够帮助您编写更加声明性、流畅的风格代码。

而不是积累起来for循环和嵌套来处理列表和集合中的数据，您能够利用这些方法更好地将逻辑组织成功能的构建块，而后将它们连接起来以建立更可读和更易于理解的实现。

ES6还为咱们提供了一些更好的数组方法，好比.find,.findIndex,.of和for..of循环！

数组循环

var officers = \[
    { id: 20, name: 'Captain Piett' },
    { id: 24, name: 'General Veers' },
    { id: 56, name: 'Admiral Ozzel' },
    { id: 88, name: 'Commander Jerjerrod' }
\];
// What you need
// \[20, 24, 56, 88\]

for循环

使用率最高，也是最基本的一种遍历方式

var officersIds = \[\];
for(var i=0,len=officers.length;i<len; i++){
    officersIds.push(officers\[i\].id);
}
console.log(officersIds); // \[20,24,56,88\]

forEach循环

forEach中传入要执行的回调函数，函数有三个参数。第一个参数为数组元素(必选)，第二个参数为数组元素索引值(可选)，第三个参数为数组自己(可选)

var officersIds = \[\];
officers.forEach(function (officer,index,array) {
    console.log(index); //0,1,2,3,
    console.log(officer); //{id: 20, name: "Captain Piett"},{id: 24, name: "General Veers"},{id: 56, name: "Admiral Ozzel"},{id: 88, name: "Commander Jerjerrod"}
    officersIds.push(officer.id);
});
console.log(officersIds); //\[20,24,56,88\]

for in循环

for...in循环可用于循环对象和数组,推荐用于循环对象,能够用来遍历JSON

var officersIds = \[\];
for(var key in officers){
    console.log(key); // 0 1 2 3 返回数组索引
    console.log(officers\[key\]); //{id: 20, name: "Captain Piett"},{id: 24, name: "General Veers"},{id: 56, name: "Admiral Ozzel"},{id: 88, name: "Commander Jerjerrod"}
    officersIds.push(officers\[key\].id);
}
console.log(officersIds); //\[20,24,56,88\]

for of循环

可循环数组和对象，推荐用于遍历数组。

for...of提供了三个新方法：

key()是对键名的遍历；
value()是对键值的遍历；
entries()是对键值对的遍历；

let arr = \['科大讯飞', '政法BG', '前端开发'\];
for (let item of arr) {  
  console.log(item); //  科大讯飞  政法BG  前端开发
}
// 输出数组索引
for (let item of arr.keys()) {  
  console.log(item);  // 0 1 2
}
// 输出内容和索引
for (let \[item, val\] of arr.entries()) {  
  console.log(item + ':' + val); //  0:科大讯飞  1：政法BG  2：前端开发
}

var officersIds = \[\];
for (var item of officers) {
    console.log(item); //{id: 20, name: "Captain Piett"},{id: 24, name: "General Veers"},{id: 56, name: "Admiral Ozzel"},{id: 88, name: "Commander Jerjerrod"}
    officersIds.push(item.id); 
}
console.log(officersIds); // \[20,24,56,88\]
// 输出数组索引
for(var item of officers.keys()){
    console.log(item); // 0 1 2 3
}
// 输出内容和索引
for (let \[item, val\] of officers.entries()) {
    console.log(item) // 0 1 2 3 输出数组索引
    console.log(val);//{id: 20, name: "Captain Piett"},{id: 24, name: "General Veers"},{id: 56, name: "Admiral Ozzel"},{id: 88, name: "Commander Jerjerrod"}
    console.log(item + ':' + val); 
}

map循环

map() 会返回一个新数组，数组中的元素为原始数组元素调用函数处理后的值。
map() 方法按照原始数组元素顺序依次处理元素。

map 不修改调用它的原数组自己。

map()中传入要执行的回调函数，函数有三个参数。第一个参数为数组元素(必选)，第二个参数为数组元素索引值(可选)，第三个参数为数组自己(可选)

var arr = \[
    {name:'a',age:'18'},
    {name:'b',age:'19'},
    {name:'c',age:'20'}
\];
arr.map(function(item,index) {
    if(item.name == 'b') {
        console.log(index)  // 1
    }
})

数组加一

var officersIds = officers.map(function (officer) {
    return officer.id
});
console.log(officersIds); //\[20,24,56,88\]

reduce

array.reduce(function callback(accumulator, currentValue, currentIndex, array){

}\[, initialValue\])

var peoples = \[
  {
    id: 10,
    name: "Poe Dameron",
    years: 14,
  },
  {
    id: 2,
    name: "Temmin 'Snap' Wexley",
    years: 30,
  },
  {
    id: 41,
    name: "Tallissan Lintra",
    years: 16,
  },
  {
    id: 99,
    name: "Ello Asty",
    years: 22,
  }
\];

输出他们的年龄总数

var totalYears = peoples.reduce(function (total, people) {
    console.log(total);
    console.log(people);
    return total + people.years;
}, 0);
// const totalYears = people.reduce((acc, people) => acc + people.years, 0);
console.log(totalYears); //30

求年龄最大的那我的

var oldest = peoples.reduce(function (oldest, people) {
    return (oldest.years || 0) > people.years ? oldest : people;
}, {});
console.log(oldest); //{id: 2, name: "Temmin 'Snap' Wexley", years: 30}
console.log(oldest.years); //82

filter

filter() 方法建立一个新的数组，新数组中的元素是经过检查指定数组中符合条件的全部元素。

array.filter(function(currentValue,index,arr){

}, thisValue)

演示

var designer = peoples.filter(function (people) {
    return people.job === "designer";
});

组合使用

var totalScore = peoples
    .filter(function (person) {
        return person.isForceUser;
    })
    .map(function (choose) {
        return choose.mathScore + choose.englishScore;
    })
    .reduce(function (total, score) {
        return total + score;
    }, 0);

Array.from()

var divs = document.querySelectorAll('div.pane');  
var text = Array.from(divs, (d) => d.textContent);  
console.log("div text:", text);

// Old, ES5 way to get array from arguments
function() {  
  var args = \[\].slice.call(arguments);
  //...
}

// Using ES6 Array.from
function() {  
  var args = Array.from(arguments);
  //..
}

var filled = Array.from(\[1,,2,,3\], (n) => n || 0);  
console.log("filled:", filled);  
// =\> \[1,0,2,0,3\]

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