javascript - JavaScript中的null和undefined有什么区别？(What is the difference between null and undefined in JavaScript?)

Question

我想知道JavaScript中null和undefined之间的区别。

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Answer 1

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

(在JavaScript中， undefined表示变量已被声明但尚未分配值，例如：)

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

null is an assignment value.

(null是赋值。)

It can be assigned to a variable as a representation of no value:

(可以将其分配给变量，以表示没有值：)

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

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.

(从前面的示例中可以明显看出， undefined和null是两种不同的类型： undefined是类型本身（未定义），而null是对象。)

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

and

(和)

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