JavaScript数组循环的几种写法包含ES6

利用 Javascript map(),reduce() 和 filter() 数组方法可以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，您可以利用这些方法更好地将逻辑组织成功能的构建块，然后将它们链接起来以创建更可读和更易于理解的实现。

ES6 还为我们提供了一些更好的数组方法，比如.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);  
// =\> \[1,0,2,0,3\]