How to check for an undefined or null variable in JavaScript

Question

We are frequently using the following code pattern in our JavaScript code  if  typeof some variable      undefined   amp  amp  some variable    null           Do something with some variable     Is there a less verbose way of checking that has the same effect   According to some forums and literature saying simply the following should have the same effect   if  some variable           Do something with some variable     Unfortunately  Firebug evaluates such a statement as error on runtime when some variable is undefined  whereas the first one is just fine for it  Is this only an  unwanted  behavior of Firebug or is there really some difference between those two ways

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As mentioned in one of the answers  you can be in luck if you are talking about a variable that has a global scope  As you might know  the variables that you define globally tend to get added to the windows object  You can take advantage of this fact so lets say you are accessing a variable called bleh  just use the double inverted operator          window  bleh      This would return a false while bleh has not been declared AND assigned a value

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Since there is no single complete and correct answer  I will try to summarize   In general  the expression   if  typeof variable      undefined   amp  amp  variable    null    cannot be simplified  because the variable might be undeclared so omitting the typeof variable      undefined  would result in ReferenceError  But  you can simplify the expression according to the context   If the variable is global  you can simplify to   if  window variable    null    If it is local  you can probably avoid situations when this variable is undeclared  and also simplify to   if  variable    null    If it is object property  you don t have to worry about ReferenceError   if  obj property    null

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This is an example of a very rare occasion where it is recommended to use    instead of      Expression somevar    null will return true for undefined and null  but false for everything else  an error if variable is undeclared    Using the    will flip the result  as expected   Modern editors will not warn for using    or    operator with null  as this is almost always the desired behavior   Most common comparisions   undeffinedVar    null        true obj undefinedProp    null    true null    null                 true 0    null                    false  0     null                  false       null                   false   Try it yourself   let undefinedVar  console table         test   undefinedVar      result  undefinedVar        null          test      undefinedProp  result     undefinedProp    null          test   null              result  null                null          test   false             result  false               null          test   0                 result  0                   null          test                     result                      null          test    0                result   0                  null

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Checking null with normal equality will also return true for undefined   if  window variable    null  alert  variable is null or undefined

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You have to differentiate between cases    Variables can be undefined or undeclared  You ll get an error if you access an undeclared variable in any context other than typeof    if typeof someUndeclaredVar    whatever     works if someUndeclaredVar     throws error   A variable that has been declared but not initialized is undefined   let foo  if  foo    evaluates to false because foo     undefined    Undefined properties   like someExistingObj someUndefProperty  An undefined property doesn t yield an error and simply returns undefined  which  when converted to a boolean  evaluates to false  So  if you don t care about  0 and false  using if obj undefProp  is ok  There s a common idiom based on this fact   value   obj prop    defaultValue   which means  if obj has the property prop  assign it to value  otherwise assign the default value defautValue    Some people consider this behavior confusing  arguing that it leads to hard-to-find errors and recommend using the in operator instead  value     prop  in obj    obj prop   defaultValue

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Both values can be easily distinguished by using the strict comparison operator   Working example at   http   www thesstech com tryme filename nullandundefined  Sample Code   function compare        var a   null    variable assigned null value     var b      undefined     if  a     b           document write  a and b have same datatype               else          document write  a and b have different datatype

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In order to understand  Let s analyze what will be the value return by the Javascript Engine when converting undefined   null and    An empty string also   You can directly check the same on your developer console     You can see all are converting to false   means All these three are assuming    lack of existence    by javascript  So you no need to explicitly check all the three in your code like below   if  a     undefined    a     null    a             console log  Nothing      else       console log  Something        Also I want to point out one more thing   What will be the result of Boolean 0    Of course false  This will create a bug in your code when 0 is a valid value in your expected result  So please make sure you check for this when you write the code

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I think the most efficient way to test for  value is null or undefined  is   if   some variable    null         some variable is either null or undefined     So these two lines are equivalent   if   typeof some variable       undefined   amp  amp  some variable     null      if   some variable    null        Note 1  As mentioned in the question  the short variant requires that some variable has been declared  otherwise a ReferenceError will be thrown  However in many use cases you can assume that this is safe   check for optional arguments   function foo       if  foo    null           check for properties on an existing object  if my obj foo    null          On the other hand typeof can deal with undeclared global variables  simply returns undefined   Yet these cases should be reduced to a minimum for good reasons  as Alsciende explained   Note 2  This - even shorter - variant is not equivalent   if    some variable          some variable is either null  undefined  0  NaN  false  or an empty string     so  if   some variable          we don t get here if some variable is null  undefined  0  NaN  false  or        Note 3  In general it is recommended to use     instead of     The proposed solution is an exception to this rule  The JSHint syntax checker even provides the eqnull option for this reason   From the jQuery style guide      Strict equality checks       should be used in favor of     The only   exception is when checking for undefined and null by way of null      Check for both undefined and null values  for some important reason   undefOrNull    null

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If you try and reference an undeclared variable  an error will be thrown in all JavaScript implementations   Properties of objects aren t subject to the same conditions  If an object property hasn t been defined  an error won t be thrown if you try and access it  So in this situation you could shorten    if  typeof myObj some property      undefined   amp  amp  myObj some property    null    to  if  myObj some property    null    With this in mind  and the fact that global variables are accessible as properties of the global object  window in the case of a browser   you can use the following for global variables   if  window some variable    null           Do something with some variable     In local scopes  it always useful to make sure variables are declared at the top of your code block  this will save on recurring uses of typeof

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If the purpose of the if statement is to check for null or undefined values before assigning a value to a variable  you can make use of the Nullish Coalescing Operator  According to the data from caniuse  it should be supported by around 85  of the browsers as of January 2021   An example of the operator is shown below  const a   some variable         This will ensure that the variable will be assigned to an empty string  or any other default value  if some variable is null or undefined  This operator is most suited for your use case  as it does not return the default value for other types of falsy value such as 0 and

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In newer JavaScript standards like ES5 and ES6 you can just say   gt  Boolean 0    false  gt  Boolean null     false  gt  Boolean undefined    false   all return false  which is similar to Python s check of empty variables  So if you want to write conditional logic around a variable  just say  if  Boolean myvar          Do something     here  null  or  empty string  or  undefined  will be handled efficiently

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whatever yyy is undefined or null  it will return true  if  typeof yyy     undefined      yyy        console log  yes      else       console log  no        yes  if    typeof yyy     undefined      yyy         console log  yes      else       console log  no        no

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I have done this using this method  save the id in some variable  var someVariable   document getElementById  someId      then use if condition  if someVariable             logic   else if someVariable             logic

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Open the Developer tools in your browser and just try the code shown in the below image

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Best way to compare undefined or null or 0 with ES5 and ES6 standards  if   Boolean some variable 1   amp  amp  Boolean some variable 2       false           do something

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Firstly you have to be very clear about what you test  JavaScript has all sorts of implicit conversions to trip you up  and two different types of equality comparator     and       A function  test val  that tests for null or undefined should have the following characteristics    test null            gt  true  test undefined       gt  true  test 0               gt  false  test 1               gt  false  test true            gt  false  test false           gt  false  test  s              gt  false  test                 gt  false   Let s see which of the ideas here actually pass our test   These work   val    null val     null    val     undefined typeof val      undefined     val    null typeof val       undefined     val     null   These do not work   typeof val       undefined    val   I created a jsperf entry to compare the correctness and performance of these approaches  Results are inconclusive for the time being as there haven t been enough runs across different browsers platforms  Please take a minute to run the test on your computer   At present  it seems that the simple val    null test gives the best performance  It s also pretty much the shortest  The test may be negated to val    null if you want the complement

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You must define a function of this form   validate   function some variable       return typeof some variable      undefined   amp  amp  some variable    null

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Similar to what you have  you could do something like  if  some variable     undefined    some variable     null       do stuff

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this is the only case in which    and    should be used  if  val    null  console log  val is null or undefined    if  val    null  console log  val is neither null nor undefined    For any other comparisons  the strict comparators      and      should be used  https   developer mozilla org en-US docs Web JavaScript Equality comparisons and sameness

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You can just check if the variable has a value or not  Meaning   if  myVariable       mayVariable is not     null   undefined   NaN   empty string        0   false      If you do not know whether a variable exists  that means  if it was declared  you should check with the typeof operator  e g   if  typeof myVariable      undefined             myVariable will get resolved and it is defined

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here s another way using the Array includes   method    undefined  null  includes value

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In ES5 or ES6 if you need check it several times you cand do    x000D   x000D  const excluded    null  undefined       x000D   x000D  if   exluded includes varToCheck    x000D       it will bee not null  not undefined and not void string x000D    x000D   x000D   x000D

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With Ramda  you can simply do R isNil yourValue  Lodash and other helper libraries have the same function

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Testing nullity  if  value    null   or non-nullity  if  value    null   is less verbose than testing the definition status of a variable   Moreover  testing if  value   or  if  obj property   to ensure the existence of your variable  or object property  fails if it is defined with a boolean false value  Caveat emptor

[javascript] How to check for an undefined or null variable in JavaScript?

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