Why is my JavaScript function sometimes not defined

Question

I call my JavaScript function  Why do I sometimes get the error  myFunction is not defined  when it is defined   For example  I ll occasionally get  copyArray is not defined  even in this example   function copyArray  pa         var la           for  var i 0  i  lt  pa length  i            la push  pa i         return la     Function prototype bind   function  po         var   method   this      var   args               Sometimes errors -- in practice I inline the function as a workaround        args   copyArray  arguments         return function                bind logic omitted for brevity              As you can see  copyArray is defined right there  so this can t be about the order in which script files load   I ve been getting this in situations that are harder to work around  where the calling function is located in another file that should be loaded after the called function  But this was the simplest case I could present  and appears to be the same problem   It doesn t happen 100  of the time  so I do suspect some kind of load-timing-related problem  But I have no idea what    Hojou  That s part of the problem   The function in which I m now getting this error is itself my addLoadEvent  which is basically a standard version of the common library function    James  I understand that  and there is no syntax error in the function  When that is the case  the syntax error is reported as well  In this case  I am getting only the  not defined  error    David  The script in this case resides in an external file that is referenced using the normal  lt script src  file js  gt  lt  script gt  method in the page s head section    Douglas  Interesting idea  but if this were the case  how could we ever call a user-defined function with confidence  In any event  I tried this and it didn t work    sk  This technique has been tested across browsers and is basically copied from the Prototype library

User · Answer

Solved by removing a  async  load      lt script type  text javascript  src     static  js my js file js      async gt  lt  script gt    changed for      lt script type  text javascript  src     static  js my js file js      gt  lt  script gt

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Verify your code with JSLint  It will usually find a ton of small errors  so the warning  JSLint may hurt your feelings  is pretty spot on

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I think your javascript code should be placed between  tag there is need of document load

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I had this function not being recognized as defined in latest Firefox for Linux  though Chromium was dealing fine with it   What happened in my case was that I had a former SCRIPT block  before the block that defined the function with problem  stated in the following way    lt SCRIPT src  mycode js   gt     That is  without the closing tag    I had to redeclare this block in the following way    lt SCRIPT src  mycode js  gt  lt  SCRIPT gt    And then what followed worked fine    weird huh

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This has probably been corrected  but    apparently firefox has a caching problem which is the cause of javascript functions not being recognized   I really don t know the specifics  but if you clear your cache that will fix the problem  until your cache is full again    not a good solution    I ve been looking around to see if firefox has a real solution to this  but so far nothing    oh not all versions  I think it may be only in some 3 6 x versions  not sure

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It shouldn t be possible for this to happen if you re just including the scripts on the page    The  copyArray  function should always be available when the JavaScript code starts executing no matter if it is declared before or after it -- unless you re loading the JavaScript files in dynamically with a dependency library  There are all sorts of problems with timing if that s the case

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My guess is  somehow the document is not fully loaded by the time the method is called  Have your code executing after the document is ready event

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A syntax error in the function -- or in the code above it -- may cause it to be undefined

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If you re changing the prototype of the built-in  function  object it s possible you re running into a browser bug or race condition by modifying a fundamental built-in object    Test it in multiple browsers to find out

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This can happen when using framesets  In one frame  my variables and methods were defined  In another  they were not  It was especially confusing when using the debugger and seeing my variable defined  then undefined at a breakpoint inside a frame

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This doesn t solve your original problem  but you could always replace the call to copyArray   with     args   Array prototype slice call arguments     More information available from Google   I ve tested the above in the following browsers  IE6  7  amp  8B2  Firefox 2 0 0 17  amp  3 0 3  Opera 9 52  Safari for Windows 3 1 2 and Google Chrome  whatever the latest version was at the time of this post  and it works across all browsers

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I m afraid  when you add a new method to a Function class  by prtotyping   you are actually adding it to all declared functions  AS WELL AS to your copyArray     In result your copyArray   function gets recursivelly self-referenced   I e  there should exist copyArray   bind   method  which is calling itself   In this case some browsers might prevent you from creating such reference loops and fire  function not defined  error   Inline code would be better solution in such case

User · Answer

Use an anonymous function to protect your local symbol table  Something like    function         function copyArray pa               Details            Function prototype bind   function   po               args   copyArray  arguments                  This will create a closure that includes your function in the local symbol table  and you won t have to depend on it being available in the global namespace when you call the function

User · Answer

Verify your code with JSLint  It will usually find a ton of small errors  so the warning  JSLint may hurt your feelings  is pretty spot on

[javascript] Why is my JavaScript function sometimes "not defined"?

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