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
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.
Update compatibility data on GitHub
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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