for...of
语句在可迭代对象(包括 Array
,Map
,Set
,String
,TypedArray
,arguments 对象等等)上创建一个迭代循环,调用自定义迭代钩子,并为每个不同属性的值执行语句
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
for (variable of iterable) {
//statements
}
variable
iterable
Array
let iterable = [10, 20, 30];
for (let value of iterable) {
value += 1;
console.log(value);
}
// 11
// 21
// 31
如果你不想修改语句块中的变量 , 也可以使用const
代替let
。
let iterable = [10, 20, 30];
for (const value of iterable) {
console.log(value);
}
// 10
// 20
// 30
String
let iterable = "boo";
for (let value of iterable) {
console.log(value);
}
// "b"
// "o"
// "o"
TypedArray
let iterable = new Uint8Array([0x00, 0xff]);
for (let value of iterable) {
console.log(value);
}
// 0
// 255
Map
let iterable = new Map([["a", 1], ["b", 2], ["c", 3]]);
for (let entry of iterable) {
console.log(entry);
}
// ["a", 1]
// ["b", 2]
// ["c", 3]
for (let [key, value] of iterable) {
console.log(value);
}
// 1
// 2
// 3
Set
let iterable = new Set([1, 1, 2, 2, 3, 3]);
for (let value of iterable) {
console.log(value);
}
// 1
// 2
// 3
arguments
对象(function() {
for (let argument of arguments) {
console.log(argument);
}
})(1, 2, 3);
// 1
// 2
// 3
迭代 DOM 元素集合,比如一个NodeList
对象:下面的例子演示给每一个 article 标签内的 p 标签添加一个 "read
" 类。
//注意:这只能在实现了NodeList.prototype[Symbol.iterator]的平台上运行
let articleParagraphs = document.querySelectorAll("article > p");
for (let paragraph of articleParagraphs) {
paragraph.classList.add("read");
}
对于for...of
的循环,可以由break
, throw continue
或return
终止。在这些情况下,迭代器关闭。
function* foo(){
yield 1;
yield 2;
yield 3;
};
for (let o of foo()) {
console.log(o);
break; // closes iterator, triggers return
}
你还可以迭代一个生成器:
function* fibonacci() { // 一个生成器函数
let [prev, curr] = [0, 1];
for (;;) { // while (true) {
[prev, curr] = [curr, prev + curr];
yield curr;
}
}
for (let n of fibonacci()) {
console.log(n);
// 当n大于1000时跳出循环
if (n >= 1000)
break;
}
生成器不应该重用,即使for...of
循环的提前终止,例如通过break
关键字。在退出循环后,生成器关闭,并尝试再次迭代,不会产生任何进一步的结果。
var gen = (function *(){
yield 1;
yield 2;
yield 3;
})();
for (let o of gen) {
console.log(o);
break;//关闭生成器
}
//生成器不应该重用,以下没有意义!
for (let o of gen) {
console.log(o);
}
你还可以迭代显式实现可迭代协议的对象:
var iterable = {
[Symbol.iterator]() {
return {
i: 0,
next() {
if (this.i < 3) {
return { value: this.i++, done: false };
}
return { value: undefined, done: true };
}
};
}
};
for (var value of iterable) {
console.log(value);
}
// 0
// 1
// 2
for...of
与for...in
的区别无论是for...in
还是for...of
语句都是迭代一些东西。它们之间的主要区别在于它们的迭代方式。
for...of
语句遍历可迭代对象定义要迭代的数据。
以下示例显示了与Array
一起使用时,for...of
循环和for...in
循环之间的区别。
Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};
let iterable = [3, 5, 7];
iterable.foo = 'hello';
for (let i in iterable) {
console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}
for (let i in iterable) {
if (iterable.hasOwnProperty(i)) {
console.log(i); // logs 0, 1, 2, "foo"
}
}
for (let i of iterable) {
console.log(i); // logs 3, 5, 7
}
Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};
let iterable = [3, 5, 7];
iterable.foo = 'hello';
每个对象将继承objCustom
属性,并且作为Array
的每个对象将继承arrCustom
属性,因为将这些属性添加到Object.prototype
和Array.prototype
。由于继承和原型链,对象iterable
继承属性objCustom
和arrCustom
。
for (let i in iterable) {
console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}
此循环仅以原始插入顺序记录iterable
对象的可枚举属性。它不记录数组元素3
, 5
, 7
或hello
,因为这些不是枚举属性。但是它记录了数组索引以及arrCustom
和objCustom
。如果你不知道为什么这些属性被迭代,array iteration and for...in
中有更多解释。
for (let i in iterable) {
if (iterable.hasOwnProperty(i)) {
console.log(i); // logs 0, 1, 2, "foo"
}
}
这个循环类似于第一个,但是它使用hasOwnProperty()
来检查,如果找到的枚举属性是对象自己的(不是继承的)。如果是,该属性被记录。记录的属性是0
, 1
, 2
和foo
,因为它们是自身的属性(不是继承的)。属性arrCustom
和objCustom
不会被记录,因为它们是继承的。
for (let i of iterable) {
console.log(i); // logs 3, 5, 7
}
该循环迭代并记录iterable
作为可迭代对象定义的迭代值,这些是数组元素 3
, 5
, 7
,而不是任何对象的属性。
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) for...of statement |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) for...of statement |
Draft |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
for...of | Chrome Full support 38 | Edge Full support 12 | Firefox
Full support
13
| IE No support No | Opera Full support 25 | Safari Full support 8 | WebView Android Full support 38 | Chrome Android Full support Yes | Edge Mobile Full support 12 | Firefox Android
Full support
14
| Opera Android Full support 25 | Safari iOS Full support 8 | Samsung Internet Android Full support Yes | nodejs Full support Yes |
async iterators | Chrome Full support 63 | Edge ? | Firefox Full support 57 | IE No support No | Opera Full support 50 | Safari ? | WebView Android Full support 63 | Chrome Android Full support 63 | Edge Mobile ? | Firefox Android Full support 57 | Opera Android Full support 50 | Safari iOS ? | Samsung Internet Android ? | nodejs ? |
Closing iterators | Chrome Full support 51 | Edge Full support Yes | Firefox Full support 53 | IE No support No | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile Full support Yes | Firefox Android Full support 53 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support Yes |