What is the difference between JS undefined and null

stay JavaScript in ,undefined Indicates that a variable has been declared but has not been assigned , for example ：

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

null It's assignment . It can be assigned to variables as a representation of no value ：

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

As can be seen from the previous example ,undefined and null There are two different types ：undefined Is itself a type （ Undefined ）, and null It's an object .

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

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