Basic JavaScript Learning--typeof,null,undefined

Source: Internet
Author: User

Null

In JavaScript, null means "nothing".

Null is a special type with only one value. Represents an empty object reference.

var person = {firstName: "John", LastName: "Doe", Age:50, Eyecolor: "Blue"}; var NULL ;d Ocument.getelementbyid (typeof person;

Undefined

In JavaScript, undefined is a variable that has no value set.

typeof A variable that has no value returns undefined.

The difference between undefined and null

document.getElementById ("Demo"). InnerHTML =typeof undefined + "<br>" +typeofnull + " <br> "+(null = = = undefined) +" <br> "+(null = = undefined);

Printing results:

Undefined

Object

False

True

I don't understand why null==undefined is true,
On Baidu to see someone said

In fact, the undefined value is derived from a null value, and the ECMAScript standard specifies that the test for equality should return true, that is,

alert (null==undefined); True


Basic JavaScript Learning--typeof,null,undefined

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