in - JavaScript 编辑

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。

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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 (ECMA-262) Relational Operators	Living Standard
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

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The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

相关链接

• for...in
• delete
• Object.prototype.hasOwnProperty()
• Reflect.has()
• 属性的可枚举性和所有权