How to check a not-defined variable in JavaScript

Question

I wanted to check whether the variable is defined or not  For example  the following throws a not-defined error    alert  x      How can I catch this error

User · Answer

The void operator returns undefined for any argument expression passed to it  so you can test against the result  actually some minifiers change your code from undefined to void 0 to save a couple of characters   For example   void 0    undefined  if  variable     void 0           variable is undefined

User · Answer

I use a small function to verify a variable has been declared  which really cuts down on the amount of clutter in my javascript files  I add a check for the value to make sure that the variable not only exists  but has also been assigned a value  The second condition checks whether the variable has also been instantiated  because if the variable has been defined but not instantiated  see example below   it will still throw an error if you try to reference it s value in your code    Not instantiated - var my variable  Instantiated - var my variable         function varExists el       if   typeof el      undefined   amp  amp  typeof el val        undefined           return true       else        return false            You can then use a conditional statement to test that the variable has been both defined AND instantiated like this     if   varExists variable name         checks that it DOES exist      or to test that it hasn t been defined and instantiated use     if   varExists variable name         checks that it DOESN T exist

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I ve often done   function doSomething variable        var undef       if variable     undef                 alert  Hey moron  define this bad boy

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An even easier and more shorthand version would be   if   x         Undefined     OR  if  typeof x      undefined           Do something since x is defined

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Another potential  solution  is to use the window object  It avoids the reference error problem when in a browser   if  window x        alert  x exists and is truthy      else       alert  x does not exist  or exists and is falsy

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The error is telling you that x doesn   t even exist  It hasn   t been declared  which is different than being assigned a value   var x     declaration x   2     assignment   If you declared x  you wouldn   t get an error  You would get an alert that says undefined because x exists has been declared but hasn   t been assigned a value   To check if the variable has been declared  you can use typeof  any other method of checking if a variable exists will raise the same error you got initially   if typeof x        undefined         alert x       This is checking the type of the value stored in x  It will only return undefined when x hasn   t been declared OR if it has been declared and was not yet assigned

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The accepted answer is correct  Just wanted to add one more option  You also can use try     catch block to handle this situation  A freaky example   var a  try       a   b   1      throws ReferenceError if b is not defined    catch  e        a   1          apply some default behavior in case of error   finally       a   a    0     normalize the result in any case     Be aware of catch block  which is a bit messy  as it creates a block-level scope  And  of course  the example is extremely simplified to answer the asked question  it does not cover best practices in error handling

User · Answer

We can check undefined as follows          var x    if  x     undefined        alert  x is undefined      else        alert  x is defined

User · Answer

I often use the simplest way   var variable  if  variable     undefined       console log  Variable is undefined      else       console log  Variable is defined        EDIT   Without initializing the variable  exception will be thrown  Uncaught ReferenceError  variable is not defined

User · Answer

Just do something like below   function isNotDefined value      return typeof value      undefined       and call it like   isNotDefined undefined     return true isNotDefined  Alireza      return false

User · Answer

Technically  the proper solution is  I believe    typeof x      undefined    You can sometimes get lazy and use  x    null   but that allows both an undefined variable x  and a variable x containing null  to return true

User · Answer

In JavaScript  null is an object  There s another value for things that don t exist  undefined  The DOM returns null for almost all cases where it fails to find some structure in the document  but in JavaScript itself undefined is the value used   Second  no  there is not a direct equivalent  If you really want to check for specifically for null  do   if  yourvar     null     Does not execute if yourvar is  undefined    If you want to check if a variable exists  that can only be done with try catch  since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent   But  to check if a variable is declared and is not undefined   if  yourvar     undefined     Any scope   Previously  it was necessary to use the typeof operator to check for undefined safely  because it was possible to reassign undefined just like a variable  The old way looked like this   if  typeof yourvar      undefined      Any scope   The issue of undefined being re-assignable was fixed in ECMAScript 5  which was released in 2009  You can now safely use     and     to test for undefined without using typeof as undefined has been read-only for some time   If you want to know if a member exists independent but don t care what its value is   if   membername  in object     With inheritance if  object hasOwnProperty  membername       Without inheritance   If you want to to know whether a variable is truthy   if  yourvar    Source

User · Answer

The only way to truly test if a variable is undefined is to do the following  Remember  undefined is an object in JavaScript   if  typeof someVar      undefined          Your variable is undefined     Some of the other solutions in this thread will lead you to believe a variable is undefined even though it has been defined  with a value of NULL or 0  for instance

User · Answer

You can also use the ternary conditional-operator    x000D   x000D  var a    hallo world   x000D  var a    a   document write  i dont know  a      document write  a       a   x000D   x000D   x000D     x000D   x000D    var a    hallo world   x000D  var a    a   document write  i dont know  a      document write  a       a   x000D   x000D   x000D

[javascript] How to check a not-defined variable in JavaScript

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