javascript check for not null

Question

Below is a code snippet  where we retrieve a form value  Before further processing check if the value is not null    var val   document FileList hiddenInfo value  alert  val is     val       this prints null which is as expected if  val    null       alert  value is   val length      this returns 4   else      alert  value  is null        Any ideas why it happens so

User · Answer

There are 3 ways to check for  quot not null quot   My recommendation is to use the Strict Not Version  1  Strict Not Version if  val     null           The Strict Not Version uses the  quot Strict Equality Comparison Algorithm quot  http   www ecma-international org ecma-262 5 1  sec-11 9 6  The     has faster performance  than the    operator because the Strict Equality Comparison Algorithm doesn t typecast values  2  Non-strict Not Version if  val     null            The Non-strict version uses the  quot Abstract Equality Comparison Algorithm quot  http   www ecma-international org ecma-262 5 1  sec-11 9 3  The    has slower performance  than the     operator because the Abstract Equality Comparison Algorithm typecasts values  3  Double Not Version if    val           The Double Not Version    has faster performance  than both the Strict Not Version     and the Non-Strict Not Version     https   jsperf com tfm-not-null 6   However  it will typecast  quot Falsey quot  values like undefined and NaN into False  http   www ecma-international org ecma-262 5 1  sec-9 2  which may lead to unexpected results  and it has worse readability because null isn t explicitly stated

User · Answer

Use     as    will get you into a world of nontransitive JavaScript truth table weirdness

User · Answer

If you want to be able to include 0 as a valid value   if    val    val     0

User · Answer

It is possibly because the value of val is actually the string  null  rather than the value null

User · Answer

This should work fine       if val   null               alert  value is   val length     -- this returns 4           else              alert  value  is null

User · Answer

this will do the trick for you  if    val        alert  this is not null     else       alert  this is null

User · Answer

This will work   if  val        alert  Not null      else       alert  Null

User · Answer

You should be using the strict not equals comparison operator     so that if the user inputs  null  then you won t get to the else

User · Answer

Check https   softwareengineering stackexchange com a 253723  if value        will evaluate to true if value is not   null undefined NaN empty string      0 false

User · Answer

It s because val is not null  but contains  null  as a string   Try to check with  null   if   null     val    For an explanation of when and why this works  see the details below

[javascript] javascript check for not null

1. Strict Not Version

2. Non-strict Not Version

3. Double Not Version

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