JavaScript checking for null vs undefined and difference between and

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How do I check a variable if it s null or undefined and what is the difference between the null and undefined  What is the difference between    and      it s hard to search Google for

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Ad 1  null is not an identifier for a property of the global object  like undefined can be   x000D   x000D  let x          undefined x000D  let y null     null x000D  let z 3        has value x000D      w          is undeclared x000D   x000D  if  x  console log  x is null or undefined    x000D  if  y  console log  y is null or undefined    x000D  if  z  console log  z is null or undefined    x000D   x000D  try   if w  0   catch e    console log  w is undeclared     x000D     typeof not throw exception for undelared variabels x000D  if typeof w      undefined   console log  w is undefined    x000D   x000D   x000D    Ad 2  The     check values and types  The    dont require same types and made implicit conversion before comparison  using  valueOf   and  toString     Here you have all  src    if        its negation             its negation

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The spec is the place to go for full answers to these questions  Here s a summary    For a variable x  you can     check whether it s null by direct comparison using      Example  x     null check whether it s undefined by either of two basic methods  direct comparison with undefined or typeof  For various reasons  I prefer typeof x      undefined   check whether it s one of null and undefined by using    and relying on the slightly arcane type coercion rules that mean x    null does exactly what you want    The basic difference between    and     is that if the operands are of different types      will always return false while    will convert one or both operands into the same type using rules that lead to some slightly unintuitive behaviour  If the operands are of the same type  e g  both are strings  such as in the typeof comparison above      and     will behave exactly the same    More reading    Angus Croll s Truth  Equality and JavaScript Andrea Giammarchi s JavaScript Coercion Demystified comp lang javascript FAQs  JavaScript Type-Conversion

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How do I check a variable if it s null or undefined      Is the variable null   if  a     null     or if  a    null     but see note below      but note the latter will also be true if a is undefined   Is it undefined   if  typeof a      undefined      or if  a     undefined     or if  a    undefined     but see note below      but again  note that the last one is vague  it will also be true if a is null   Now  despite the above  the usual  way to check for those is to use the fact that they re falsey   if   a            a  is falsey  which includes  undefined  and  null          and       and  0   and  NaN   and  of course   false       This is defined by ToBoolean in the spec         and what is the difference between the null and undefined    They re both values usually used to indicate the absence of something  undefined is the more generic one  used as the default value of variables until they re assigned some other value  as the value of function arguments that weren t provided when the function was called  and as the value you get when you ask an object for a property it doesn t have  But it can also be explicitly used in all of those situations   There s a difference between an object not having a property  and having the property with the value undefined  there s a difference between calling a function with the value undefined for an argument  and leaving that argument off entirely    null is slightly more specific than undefined  It s a blank object reference  JavaScript is loosely typed  of course  but not all of the things JavaScript interacts with are loosely typed  If an API like the DOM in browsers needs an object reference that s blank  we use null  not undefined  And similarly  the DOM s getElementById operation returns an object reference nbsp  mdash  either a valid one  if it found the DOM element   or null  if it didn t    Interestingly  or not   they re their own types  Which is to say  null is the only value in the Null type  and undefined is the only value in the Undefined type      What is the difference between      and         The only difference between them is that    will do type coercion to try to get the values to match  and     won t  So for instance  1     1 is true  because  1  coerces to 1  But  1      1 is false  because the types don t match    1      1 is true   The first  real  step of     is  Are the types of the operands the same   and if the answer is  no   the result is false  If the types are the same  it does exactly what    does   Type coercion uses quite complex rules and can have surprising results  for instance        0 is true    More in the spec    Abstract Equality Comparison      also called  loose  equality  Strict Equality Comparison

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If your  logical  check is for a negation     and you want to capture both JS null and undefined   as different Browsers will give you different results  you would use the less restrictive comparison  e g    var ItemID   Item get id    if  ItemID    null       do stuff     This will capture both null and undefined

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The difference is subtle   In JavaScript an undefined variable is a variable that as never been declared  or never assigned a value  Let s say you declare var a  for instance  then a will be undefined  because it was never assigned any value   But if you then assign a   null  then a will now be null  In JavaScript null is an object  try typeof null in a JavaScript console if you don t believe me   which means that null is a value  in fact even undefined is a value    Example   var a  typeof a          gt   undefined   a   null  typeof null       gt   object    This can prove useful in function arguments  You may want to have a default value  but consider null to be acceptable  In which case you may do   function doSomething first  second  optional        if  typeof optional      undefined             optional    three                do something     If you omit the optional parameter doSomething 1  2  thenoptional will be the  three  string but if you pass doSomething 1  2  null  then optional will be null   As for the equal    and strictly equal     comparators  the first one is weakly type  while strictly equal also checks for the type of values  That means that 0     0  will return true  while 0      0  will return false  because a number is not a string   You may use those operators to check between undefined an null  For example   null     null                gt  true undefined     undefined      gt  true undefined     null           gt  false undefined    null            gt  true   The last case is interesting  because it allows you to check if a variable is either undefined or null and nothing else   function test val        return val    null    test null             gt  true test undefined        gt  true

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How do I check a variable if it s null or undefined   just check if a variable has a valid value like this     if variable    it will return true if variable does t contain     null undefined 0 false     an empty string  NaN

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undefined  It means the variable is not yet intialized     Example    var x  if x     you can check like this      code      equals      It only check value is equals not datatype    Example    var x   true  var y   new Boolean true   x    y     returns true   Because it checks only value    Strict Equals       Checks the value and datatype should be same    Example    var x   true  var y   new Boolean true   x   y    returns false    Because it checks the datatype x is a primitive type and y is a boolean object

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Try With Different Logic  You can use bellow code for check all four 4  condition for validation like not null  not blank  not undefined and not zero only use this code      variable    in javascript and jquery   function myFunction     var data     The Values can be like as null  blank  undefined  zero you can test  if     data           If data has valid value     alert  data   data      else          If data has null  blank  undefined  zero etc      alert  data is   data

[javascript] JavaScript checking for null vs. undefined and difference between == and ===

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