in

如果指定的属性在指定的对象或其原型链中,则in 运算符返回true

语法

prop in object

参数

prop
一个字符串类型或者 symbol 类型的属性名或者数组索引(非symbol类型将会强制转为字符串)。
objectName
检查它(或其原型链)是否包含具有指定名称的属性的对象。

描述

下面的例子演示了一些 in 运算符的用法。

// 数组
var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
0 in trees        // 返回true
3 in trees        // 返回true
6 in trees        // 返回false
"bay" in trees    // 返回false (必须使用索引号,而不是数组元素的值)

"length" in trees // 返回true (length是一个数组属性)

Symbol.iterator in trees // 返回true (数组可迭代,只在ES2015+上有效)


// 内置对象
"PI" in Math          // 返回true

// 自定义对象
var mycar = {make: "Honda", model: "Accord", year: 1998};
"make" in mycar  // 返回true
"model" in mycar // 返回true

in右操作数必须是一个对象值。例如,你可以指定使用String构造函数创建的字符串,但不能指定字符串文字。

var color1 = new String("green");
"length" in color1 // 返回true
var color2 = "coral";
"length" in color2 // 报错(color2不是对象)

对被删除或值为 undefined 的属性使用in

如果你使用 delete 运算符删除了一个属性,则 in 运算符对所删除属性返回 false

var mycar = {make: "Honda", model: "Accord", year: 1998};
delete mycar.make;
"make" in mycar;  // 返回false

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
delete trees[3];
3 in trees; // 返回false

如果你只是将一个属性的值赋值为undefined,而没有删除它,则 in 运算仍然会返回true

var mycar = {make: "Honda", model: "Accord", year: 1998};
mycar.make = undefined;
"make" in mycar;  // 返回true
var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
trees[3] = undefined;
3 in trees; // 返回true

继承属性

如果一个属性是从原型链上继承来的,in 运算符也会返回 true

"toString" in {}; // 返回true

规范

Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
Relational Operators
Draft  
ECMAScript 2015 (6th Edition, ECMA-262)
Relational Operators
Standard  
ECMAScript 5.1 (ECMA-262)
The in Operator
Standard  
ECMAScript 3rd Edition (ECMA-262)
The in Operator
Standard Initial definition. Implemented in JavaScript 1.4.

浏览器兼容

Update compatibility data on GitHub
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
in Chrome Full support 1 Edge Full support 12 Firefox Full support 1 IE Full support 5.5 Opera Full support Yes Safari Full support Yes WebView Android Full support 1 Chrome Android Full support 18 Firefox Android Full support 4 Opera Android Full support Yes Safari iOS Full support Yes Samsung Internet Android Full support 1.0 nodejs Full support Yes

Legend

Full support  
Full support

相关链接