When is null or undefined used in JavaScript

Question

I am really confused as to when JavaScript returns null or undefined  Also different browsers seem to be returning these differently   Could you please give some examples of null undefined with the browsers that return them   While I am now clear on the undefined aspect  I am still not 100  clear on null  Is it similar to a blank value   E g  You have a text box which does not have any value set  Now when you try to access its value  will it be null or undefined and are they similar

User · Answer

A property  when it has no definition  is undefined   null is an object  It s type is null  undefined is not an object  its type is undefined    This is a good article explaining the difference and also giving some examples   null vs undefined

User · Answer

You get undefined for the various scenarios   You declare a variable with var but never set it   var foo   alert foo     undefined    You attempt to access a property on an object you ve never set   var foo       alert foo bar     undefined   You attempt to access an argument that was never provided   function myFunction  foo      alert foo     undefined      As cwolves pointed out in a comment on another answer  functions that don t return a value   function myFunction        alert myFunction      undefined   A null usually has to be intentionally set on a variable or property  see comments for a case in which it can appear without having been set   In addition a null is of type object and undefined is of type undefined   I should also note that null is valid in JSON but undefined is not   JSON parse undefined     syntax error JSON parse null     null

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I find that some of these answers are vague and complicated  I find the best way to figure out these things for sure is to just open up the console and test it yourself   var x   x    null               true x    undefined          true x     null              false x     undefined         true  var y   null   y    null               true y    undefined          true y     null              true y     undefined         false  typeof x                 undefined  typeof y                 object   var z    abc  null    z abc    null           true z abc    undefined      true z abc     null          true z abc     undefined     false  z xyz    null           true z xyz    undefined      true z xyz     null          false z xyz     undefined     true  null   1                throws error  invalid left hand assignment undefined   1           works fine  this can cause some problems   So this is definitely one of the more subtle nuances of JavaScript  As you can see  you can override the value of undefined  making it somewhat unreliable compared to null  Using the    operator  you can reliably use null and undefined interchangeably as far as I can tell  However  because of the advantage that null cannot be redefined  I might would use it when using      For example  variable    null will ALWAYS return false if variable is equal to either null or undefined  whereas variable    undefined will return false if variable is equal to either null or undefined UNLESS undefined is reassigned beforehand   You can reliably use the     operator to differentiate between undefined and null  if you need to make sure that a value is actually undefined  rather than null    According to the ECMAScript 5 spec    Both Null and Undefined are two of the six built in types    4 3 9 undefined value     primitive value used when a variable has not been assigned a value   4 3 11 null value     primitive value that represents the intentional absence of any object value

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Regarding this topic the specification  ecma-262  is quite clear   I found it really useful and straightforward  so that I share it  - Here you will find Equality algorithm - Here you will find Strict equality algorithm  I bumped into it reading  Abstract equality  strict equality  and same value  from mozilla developer site  section sameness   I hope you find it useful

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I might be missing something  but afaik  you get undefined only  Update  Ok  I missed a lot  trying to complete   You get undefined         when you try to access properties of an object that don t exist   var a      a foo    undefined       when you have declared a variable but not initialized it   var a     a is undefined       when you access a parameter for which no value was passed   function foo  a  b           something    foo 42      b inside foo is undefined       when a function does not return a value   function foo       var a   foo       a is undefined     It might be that some built-in functions return null on some error  but if so  then it is documented  null is a concrete value in JavaScript  undefined is not     Normally you don t need to distinguish between those  Depending on the possible values of a variable  it is sufficient to use if variable  to test whether a value is set or not  both  null and undefined evaluate to false       Also different browsers seem to be returning these differently    Please give a concrete example

User · Answer

The DOM methods getElementById    nextSibling    childNodes n   parentNode   and so on return null  defined but having no value  when the call does not return a node object    The property is defined  but the object it refers to does not exist   This is one of the few times you may not want to test for equality-  if x   undefined  will be true for a null value  but if x   undefined  will be true  only  for  values that are not either undefined or null

[javascript] When is null or undefined used in JavaScript?

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