ES6 Array方法

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)，第三个参数为数组本身(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,可以用来遍历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本身。

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)，第三个参数为数组本身(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

求年龄最大的那个人

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() 方法创建一个新的数组，新数组中的元素是通过检查指定数组中符合条件的所有元素。

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]

欢迎评论回复更多你认识的语法糖。

kujian / daily-interview-question

JavaScript数组循环的各种写法一览 #4