JavaScript 中的 null 和 undefined 有什么区别? [英] What is the difference between null and undefined in JavaScript?

问题描述

我想知道 JavaScript 中的 null 和 undefined 有什么区别.

I want to know what the difference is between null and undefined in JavaScript.

推荐答案

在 JavaScript 中，undefined 表示变量已经声明但尚未赋值，例如:

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

var testVar;
alert(testVar); //shows undefined
alert(typeof testVar); //shows undefined

null 是一个赋值.可以将其分配给变量作为无值的表示:

null is an assignment value. It can be assigned to a variable as a representation of no value:

var testVar = null;
alert(testVar); //shows null
alert(typeof testVar); //shows object

从前面的例子中，很明显 undefined 和 null 是两种不同的类型: undefined 是一个类型本身(未定义)而null 是一个对象.

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

null === undefined // false
null == undefined // true
null === null // true

和

null = 'value' // ReferenceError
undefined = 'value' // 'value'

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