Check if object exists in JavaScript

Question

How do I verify the existence of an object in JavaScript   The following works   if   null     alert  GOT HERE      But this throws an Error   if   maybeObject     alert  GOT HERE      The Error       maybeObject is not defined

User · Answer

I used to just do a if maybeObject  as the null check in my javascripts    if maybeObject       alert  GOT HERE        So only if maybeObject - is an object  the alert would be shown  I have an example in my site   https   sites google com site javaerrorsandsolutions home javascript-dynamic-checkboxes

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I ve just tested the typeOf examples from above and none worked for me  so instead I ve used this    x000D   x000D      btnAdd   document getElementById  elementNotLoadedYet    x000D      if  btnAdd    x000D         btnAdd textContent    Some text here   x000D        else   x000D        alert  not detected     x000D        x000D   x000D   x000D

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You can safely use the typeof operator on undefined variables   If it has been assigned any value  including null  typeof will return something other than undefined  typeof always returns a string   Therefore  if  typeof maybeObject     undefined        alert  GOT THERE

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There are a lot of half-truths here  so I thought I make some things clearer   Actually you can t accurately tell if a variable exists  unless you want to wrap every second line into a try-catch block    The reason is Javascript has this notorious value of undefined which strikingly doesn t mean that the variable is not defined  or that it doesn t exist undefined     not defined  var a  alert typeof a      undefined  declared without a value  alert typeof b      undefined  not declared    So both a variable that exists and another one that doesn t can report you the undefined  type   As for  Kevin s misconception  null    undefined  It is due to type coercion  and it s the main reason why Crockford keeps telling everyone who is unsure of this kind of thing to always use strict equality operator     to test for possibly falsy values  null     undefined gives you what you might expect  Please also note  that foo    null can be an effective way to check if a variable is neither undefined nor null  Of course you can be explicit  because it may help readability   If you restrict the question to check if an object exists  typeof o     object  may be a good idea  except if you don t consider arrays objects  as this will also reported to be the type of object which may leave you a bit confused  Not to mention that typeof null will also give you object which is simply wrong   The primal area where you really should be careful about typeof  undefined  null  unknown and other misteries are host objects  They can t be trusted  They are free to do almost any dirty thing they want  So be careful with them  check for functionality if you can  because it s the only secure way to use a feature that may not even exist

User · Answer

If that s a global object  you can use if   window maybeObject

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for me this worked for a DOM-object   if document getElementsById  IDname   length    0       alert  object exist

User · Answer

You could use  typeof    if typeof maybeObject     undefined       alert  GOT HERE

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if  n     Object n           code

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If you care about its existence only   has it been declared      the approved answer is enough     if  typeof maybeObject     undefined        alert  GOT THERE        If you care about it having an actual value  you should add    if  typeof maybeObject     undefined   amp  amp  maybeObject    null        alert  GOT THERE        As typeof  null       object   e g  bar     x  1  y  2  z  null   typeof  bar z       object   typeof  bar not present       undefined     this way you check that it s neither null or undefined  and since typeof does not error if value does not exist plus  amp  amp  short circuits  you will never get a run-time error     Personally  I d suggest adding a helper fn somewhere  and let s not trust typeof        function exists data      data     null  amp  amp  data     undefined    if  exists  maybeObject          alert  Got here

User · Answer

Or  you can all start using my exclusive exists   method instead and be able to do things considered impossible  i e    Things like  exists  blabla    or even   exists  foreignObject guessedProperty guessNext propertyNeeded   are also possible

User · Answer

Think it s easiest like this      if myobject or myvar      alert  it exists    else    alert  what the hell you ll talking about

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if  maybeObject     undefined    alert  Got here

User · Answer

zero and null are implicit pointers   If you arn t doing arithmetic  comparing  or printing  0  to screen there is no need to actually type it   Its implicit   As in implied   Typeof is also not required for the same reason   Watch   if obj  console log  exists     I didn t see request for a not or else there for it is not included as   As much as i love extra content which doesn t fit into the question   Lets keep it simple

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The thread was opened quite some time ago  I think in the meanwhile the usage of a ternary operator is the simplest option   maybeObject   console log maybeObject id

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You can use   if  typeof objectName     object           do something

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set Textbox value to one frame to inline frame using div alignmnt tabbed panel  So first of all  before set the value we need check selected tabbed panels frame available or not using following codes   Javascript Code                                               lt script gt   function set TextID                                 try                                               if  parent frames  entry                                                              alert  Frame object not found                                                                  else                                                               var setText document getElementById  formx   value                                  parent frames  entry   document getElementById  form id   value setText                                                            if  parent frames  education                                                              alert  Frame object not found                                                                   else                                                               var setText document getElementById  formx   value                                  parent frames  education   document getElementById  form id   value setText                                                            if  parent frames  contact                                                              alert  Frame object not found                                                                   else                                                               var setText document getElementById  formx   value                                  parent frames  contact   document getElementById  form id   value setText                                                          catch exception                        lt  script gt

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Apart from checking the existence of the object variable you may want to provide a  worst case  output or at least trap it into an alert so it doesn t go unnoticed   Example of function that checks  provides alternative  and catch errors   function fillForm obj      try       var output      output    typeof obj      undefined     obj           return  output          catch  err           If an error was thrown  sent it as an alert        to help with debugging any problems     alert err toString            If the obj doesn t exist or it s empty         I want to fill the form with        return              catch End      fillForm End   I created this also because the object I was passing to it could be  x     x m    x m z   and  typeof x m z  would fail with an error if x m did not exist   I hope it helps   BTW  I am novice with JS

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Two ways   typeof for local variables  You can test for a local object using typeof   if  typeof object      undefined        window for global variables  You can test for a global object  one defined on the global scope  by inspecting the window object   if  window FormData

[javascript] Check if object exists in JavaScript

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