How can I determine if a variable is undefined or null

Question

How do I determine if variable is undefined or null   My code is as follows   var EmpName      div esd-names div name   attr  class    if EmpName     undefined         DO SOMETHING       lt div id  esd-names  gt     lt div id  name  gt  lt  div gt   lt  div gt      But if I do this  the JavaScript interpreter halts execution

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You can check if the value is undefined or null by simply using typeof   if typeof value     undefined

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Calling typeof null returns a value of    object     as the special value null is considered to be an empty object reference  Safari through version 5 and Chrome through version 7 have a quirk where calling typeof on a regular expression returns    function    while all other browsers return    object

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jQuery check element not null   var dvElement       dvElement     if  dvElement length   gt  0           Do something   else         Else do something else

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Let s look at this     nbsp   let apple     Only declare the variable as apple alert apple      undefined   In the above  the variable is only declared as apple  In this case  if we call method alert it will display undefined   nbsp      let apple   null     Declare the variable as apple and initialized but the value is null       alert apple      null    In the second one it displays null  because variable of apple value is null   So you can check whether a value is undefined or null   if apple     undefined    apple     null           Can use variable without any error

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if  typeof EmpName     undefined   amp  amp  EmpName      will evaluate to true if value is not    null undefined NaN empty string      0 false

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Combining the above answers  it seems the most complete answer would be   if  typeof variable      undefined     variable     null           Do stuff     This should work for any variable that is either undeclared or declared and explicitly set to null or undefined   The boolean expression should evaluate to false for any declared variable that has an actual non-null value

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if  variable    null           Do stuff  will only match null or undefined  this won t match false

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The standard way to catch null and undefined simultaneously is this   if  variable    null            do something      --which is 100  equivalent to the more explicit but less concise   if  variable     undefined    variable     null            do something      When writing professional JS  it s taken for granted that type equality and the behavior of    vs     is understood  Therefore we use    and only compare to null     Edit again  The comments suggesting the use of typeof are simply wrong  Yes  my solution above will cause a ReferenceError if the variable doesn t exist  This is a good thing  This ReferenceError is desirable  it will help you find your mistakes and fix them before you ship your code  just like compiler errors would in other languages  Use try catch if you are working with input you don t have control over   You should not have any references to undeclared variables in your code

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var x  if  x     undefined        alert   only declared  but not defined       if  typeof y      undefined         alert   not even declared         You can only use second one  as it will check for both definition and declaration

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Probably the shortest way to do this is   if EmpName    null       DO SOMETHING         Here is proof    x000D   x000D  function check EmpName    x000D    if EmpName    null    return true     x000D    return false  x000D    x000D   x000D  var log    t a    gt  console log    t  - gt    check a      x000D   x000D  log  null   null   x000D  log  undefined   undefined   x000D  log  NaN   NaN   x000D  log            x000D  log            x000D  log            x000D  log   1     1    x000D  log   0     0    x000D  log                x000D  log  true   true   x000D  log  false   false   x000D  log   true     true    x000D  log   false     false    x000D  log  Infinity   Infinity   x000D  log  -Infinity   -Infinity   x000D  log  1   1   x000D  log  0   0   x000D  log  -1   -1   x000D  log   1     1    x000D  log   0     0    x000D  log   -1     -1    x000D   x000D      void 0  case x000D  console log  --- n true  is    true   x000D  console log   void 0  is    void 0   x000D  log void 0 void 0       void 0  is  undefined   x000D   x000D   x000D    And here are more details about     source here     BONUS  reason why     is more clear than     look on agc answer

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The shortest and easiest   if  EmpName        DO SOMETHING     this will evaluate true if EmpName is    null undefined NaN empty string      0 false

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With the solution below    const getType    val    gt  typeof val      undefined      val   null   typeof val  const isDeepEqual    a  b    gt  getType a      getType b    console log isDeepEqual 1  1       true console log isDeepEqual null  null       true console log isDeepEqual              true console log isDeepEqual 1   1        false etc      I m able to check for the following    null  undefined  NaN empty string       0 false

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var i   if  i     null    typeof i      undefined         console log i   i is undefined or null     else       console log i   i has some value

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null    undefined      true   null     undefined     false   Because     checks for both the type and value  Type of both are different but value is the same

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The foo    null check should do the trick and resolve the  undefined OR null  case in the shortest manner   Not considering  foo is not declared  case   But people who are used to have 3 equals  as the best practice  might not accept it  Just look at eqeqeq or triple-equals rules in eslint and tslint     The explicit approach  when we are checking if a variable is undefined or null separately  should be applied in this case  and my contribution to the topic  27 non-negative answers for now   is to use void 0 as both short and safe way to perform check for undefined   Using foo     undefined is not safe because undefined is not a reserved word and can be shadowed  MDN   Using typeof      undefined  check is safe  but if we are not going to care about foo-is-undeclared case the following approach can be used   if  foo     void 0    foo     null

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In JavaScript  as per my knowledge  we can check an undefined  null or empty variable like below  if  variable     undefined      if  variable     null      if  variable             Check all conditions  if variable     undefined    variable     null    variable

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Best way   if typeof variable    undefined     variable   null        do your stuff

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You can simply use the following  I know there are shorter ways to do this  but this may make it easier to visually observe  at least for others looking at the code    if  x     null    x     undefined        Add your response code here  etc

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I still think the best safe way to test these two conditions is to cast the value to a string   var EmpName      div esd-names div name   attr  class        Undefined check if  Object prototype toString call EmpName        object Undefined            Do something with your code       Nullcheck if  Object prototype toString call EmpName        object Null            Do something with your code

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I know I m 10 years late  But I ll leave my answer here just in case anybody needs a short method  Installing Lodash in your project could be useful because of the helper functions that could come in handy in these situations  Using ES6 modules  import would look like this  import isNull from  lodash isNull    import isUndefined from  lodash isUndefined    import isNil from  lodash isNil    Would be better if only the used functions are imported  Lodash s isNull checks if value is null  const value   null     if isNull value            do something if null     lodash s isUndefined checks if value is undefined  const value   undefined   if isUndefined value           do something if undefined     isNil Checks if value is null OR undefined  I prefer this method over other two  because it checks both undefined and null

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I ve just had this problem i e  checking if an object is null  I simply use this   if  object           Your code     For example   if  document getElementById  enterJob          document getElementById  enterJob   className      current

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If the variable you want to check is a global  do  if  window yourVarName           Your code here     This way to check will not throw an error even if the yourVarName variable doesn t exist   Example  I want to know if my browser supports History API  if  window history        history back        How this works   window is an object which holds all global variables as its properties  and in JavaScript it is legal to try to access a non-existing object property  If history doesn t exist then window history returns undefined  undefined is falsey  so code in an if undefined    block won t run

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Since you are using jQuery  you can determine whether a variable is undefined or its value is null by using a single function   var s     undefined jQuery isEmptyObject s      will return true   s   null     defined as null jQuery isEmptyObject s      will return true      usage if jQuery isEmptyObject s        alert  Either variable  s is undefined or its value is null     else       alert  variable  s has value     s      s    something      defined with some value jQuery isEmptyObject s      will return false

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if you create a function to check it   export function isEmpty  v     if  typeof v      undefined        return true      if  v     null       return true      if  typeof v      object   amp  amp  Object keys v  length     0       return true       if  Array isArray v   amp  amp  v length     0       return true       if  typeof v      string   amp  amp  v trim   length     0       return true      return false

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You can use the qualities of the abstract equality operator to do this   if  variable    null          your code here      Because null    undefined is true  the above code will catch both null and undefined

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I run this test in the Chrome console  Using  void 0  you can check undefined   var c  undefined if  c     void 0  alert       output    undefined var c   1     output    undefined if  c     void 0  alert       output     undefined    check c value  c    output    1 if  c     void 0  alert       output    undefined c   undefined     output    undefined if  c     void 0  alert       output     undefined

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The easiest way to check is   if  variable         If the variable is null or undefined then execution of code will enter here

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jQuery attr   function returns either a blank string or the actual value  and never null or undefined   The only time it returns undefined is when your selector didn t return any element   So you may want to test against a blank string  Alternatively  since blank strings  null and undefined are false-y  you can just do this   if   EmpName      do something

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To test if a variable is null or undefined I use the below code       if typeof sVal      undefined     sVal     null    sVal                console log  variable is undefined or null

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if x  null  is a bad idea in JavaScript  Judge with      - it may cause an unexpected type coercion  and it can t be read by CoffeeScript   never use      or      in condition judgment   if x  will be better  but be careful with 0 and     It will be treated as false  not the equal method with     null  is true     See JavaScript Best Practices

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I ve come to write my own function for this  JavaScript is weird   It is usable on literally anything   Note that this also checks if the variable contains any usable values  But since this information is usually also needed  I think it s worth posting   Please consider leaving a note   function empty v        let type   typeof v      if  type      undefined             return true            if  type      boolean             return  v            if  v     null            return true            if  v     undefined            return true            if  v instanceof Array            if  v length  lt  1                return true                  else if  type      string             if  v length  lt  1                return true                    if  v      0                 return true                  else if  type      object             if  Object keys v  length  lt  1                return true                  else if  type      number             if  v     0                return true                      return false      TypeScript-compatible     This function should do exactly the same thing like PHP s empty   function  see RETURN VALUES   Considers undefined  null  false  0  0 0   0         as empty    0 0   NaN       true are considered non-empty

[javascript] How can I determine if a variable is 'undefined' or 'null'?

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