Let’s talk about the similarities first.

让我们先谈一下相似之处。

null and undefined are JavaScript primitive types.

null和undefined是JavaScript基本类型。

The meaning of undefined is to say that a variable has declared, but it has no value assigned.

undefined的意思是说已经声明了一个变量，但是没有赋值。

let age //age is undefined

let age = null //age is null

Note: accessing a variable that’s not been declared will raise a ReferenceError: <variable> is not defined error, but this does not mean it’s undefined.

注意：访问未声明的变量将引发ReferenceError: <variable> is not defined错误，但这并不意味着undefined 。

How do you check if a variable is null? Use the comparison operator, for example age === null

如何检查变量是否为空？ 使用比较运算符，例如age === null

Same for undefined: age === undefined

与未定义相同： age === undefined

In both cases, you can check for:

在这两种情况下，您都可以检查：

if ( ! age ) {}

and this will be matching both null and undefined.

这将匹配null和undefined 。

You can also use the typeof operator:

您还可以使用typeof运算符：

let agetypeof age //'undefined'

although null is evaluated as an object, even though it is a primitive type:

尽管null被评估为一个对象，即使它是原始类型：

let age = nulltypeof age //'object'

翻译自: