什么时候“未定义”等于JavaScript中的“null”？

Question

I've been watching some videos from Tut+ about Js.They said that sometimes "undefined" is equal to "null". So, when does this happen ?

Answer 1

undefined == null
// => true

undefined === null
// => false

== tests for equality, === tests for identity (or strict equality). If in doubt, use === .

Answer 2

Just to add on , This question is somehow answered already, check here

You can just check if the variable has a truthy value or not. That means

 if( value ) {

 }

will evaluate to true if value is not:

null

undefined

NaN

empty string ("")

0

false

Answer 3

this is because of the poor typing in JS

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

great!!!

good practice is not to use null at all in js. You cannot not get rid of undefined because it is built in. if you access an undefined variable its === undefinded, but not equals null, right? So to not get confused, just stop using it. define it as undefined, but not as null ;)

so don't use null