Testing if value is a function

Question

I need to test whether the value of a form s onsubmit is a function   The format is typically onsubmit  return valid       Is there a way to tell if this is a function  and if it s callable   Using typeof just returns that it s a string  which doesn t help me much   EDIT  Of course  I understand that  return valid     is a string   I ve replaced it down to  valid      and even  valid      I want to know if either of those is a function   EDIT  Here s some code  which may help explain my problem      a button   parents  form   submit function         var submit function      a button   parents  form   attr  onsubmit        if   submit function  amp  amp  typeof  submit function replace  return             function              return eval submit function replace  return               else           alert  onSubmit is not a function  n nIs the script included     return false               EDIT 2  Here s the new code   It seems that I still have to use an eval  because calling form submit   doesn t fire existing onsubmits   var formObj      a button   parents  form    formObj submit function         if   formObj 0  onsubmit  amp  amp  typeof  formObj onsubmit       function              return eval formObj attr  onsubmit   replace  return               else           alert  onSubmit is not a function  n nIs the script included             return false               Suggestions on possibly how to do this better

User · Answer

You could simply use the typeof operator along with a ternary operator for short   onsubmit  return typeof valid    function    valid     true     If it is a function we call it and return it s return value  otherwise just return true  Edit   I m not quite sure what you really want to do  but I ll try to explain what might be happening   When you declare your onsubmit code within your html  it gets turned into a function and thus its callable from the JavaScript  world   That means that those two methods are equivalent   HTML   lt form onsubmit  return valid       gt  JavaScript  myForm onsubmit   function     return valid         These two will be both functions and both will be callable  You can test any of those using the typeof operator which should yeld the same result   function    Now if you assign a string to the  onsubmit  property via JavaScript  it will remain a string  hence not callable  Notice that if you apply the typeof operator against it  you ll get  string  instead of  function    I hope this might clarify a few things  Then again  if you want to know if such property  or any identifier for the matter  is a function and callable  the typeof operator should do the trick  Although I m not sure if it works properly across multiple frames   Cheers

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You can try modifying this technique to suit your needs    function isFunction        var functionName   window prompt  Function name         var isDefined   eval   typeof     functionName        function           if  isDefined       eval functionName              else      alert  Function     functionName     does not exist        function anotherFunction        alert  message from another function

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form onsubmit will always be a function when defined as an attribute of HTML the form element  It s some sort of anonymous function attached to an HTML element  which has the this pointer bound to that FORM element and also has a parameter named event which will contain data about the submit event   Under these circumstances I don t understand how you got a string as a result of a typeof operation  You should give more details  better some code   Edit  as a response to your second edit    I believe the handler attached to the HTML attribute will execute regardless of the above code  Further more  you could try to stop it somehow  but  it appears that FF 3  IE 8  Chrome 2 and Opera 9 are executing the HTML attribute handler in the first place and then the one attached  I didn t tested with jQuery though  but with addEventListener and attachEvent   So    what are you trying to accomplish exactly   By the way  your code isn t working because your regular expression will extract the string  valid      which is definitely not a function

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if   window onsubmit                 else        alert  Function does not exist

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A simple check like this will let you know if it exists defined   if  this onsubmit         do stuff

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If it s a string  you could assume   hope it s always of the form   return SomeFunction arguments     parse for the function name  and then see if that function is defined using   if  window functionName             do stuff

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I m replacing a submit button with an   anchor link  Since calling   form submit   does not activate   onsubmit s  I m finding it  and   eval  ing it myself  But I d like to   check if the function exists before   just eval  ing what s there      gms8994    lt script type  text javascript  gt  function onsubmitHandler         alert  running onsubmit handler        return true    function testOnsubmitAndSubmit f        if  typeof f onsubmit      function                onsubmit is executable  test the return value         if  f onsubmit                     onsubmit returns true  submit the form             f submit                       lt  script gt    lt form name  theForm  onsubmit  return onsubmitHandler     gt   lt a href     onclick       testOnsubmitAndSubmit document forms  theForm         return false    gt  lt  a gt   lt  form gt    EDIT   missing parameter f in function testOnsubmitAndSubmit  The above should work regardless of whether you assign the onsubmit HTML attribute or assign it in JavaScript   document forms  theForm   onsubmit   onsubmitHandler

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Isn t typeof xxx      function  the best and the fastest  I made an bench in wich you can try it out  compared to instanceof and  underscore  Its just seems to be faster than instanceof  using chrome  It won t trow an error if the variable is not defined  Here a bench  https   jsbench me qnkf076cqb 1

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Make sure you are calling typeof on the actual function  not a string literal   function x          console log  hi        typeof  x      returns  string   typeof x     returns  function

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Try   if  this onsubmit instanceof Function           do stuff

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Well   return valid     is a string  so that s correct   If you want to check if it has a function attached instead  you could try this   formId onsubmit   function              if typeof formId onsubmit     function      alert  it s a function

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using a string based variable as example and making use  instanceof Function  You register the function  assign the variable   check the variable is the name of function   do pre-process    assign the function to new var   then call the function    function callMe       alert  You rang        var value    callMe    if  window value  instanceof Function            do pre-process stuff        FYI the function has not actually been called yet     console log  callable function          now call function    var fn   window value      fn

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You can always use one of the typeOf functions on JavaScript blogs such as Chris West s   Using a definition such as the following for the typeOf   function would work   function typeOf o  return    toString call o  slice 8 -1     This function  which is declared in the global namespace  can be used like this   alert  onsubmit is a     typeOf elem onsubmit      If it is a function   Function  will be returned   If it is a string   String  will be returned   Other possible values are shown here

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This should be a function  because in certain JavaScript engines  V8  for    example  try block kills many optimizations   function isFunction func           For some reason  function constructor doesn t accept anonymous functions         Also  this check finds callable objects that aren t function  such as         regular expressions in old WebKit versions   as according to EcmaScript        specification  any callable object should have typeof set to function      if  typeof func      function           return true         If the function isn t a string  it s probably good idea to return false         as eval cannot process values that aren t strings      if  typeof func      string           return false         So  the value is a string  Try creating a function  in order to detect        syntax error      try              Create a function with string func  in order to detect whatever it s            an actual function  Unlike examples with eval  it should be actually            safe to use with any string  provided you don t call returned value           Function func          return true           catch  e               While usually only SyntaxError could be thrown  unless somebody            modified definition of something used in this function  like            SyntaxError or Function  it s better to prepare for unexpected          if    e instanceof SyntaxError                 throw e                    return false

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I think the source of confusion is the distinction between a node s attribute and the corresponding property   You re using      a button   parents  form   attr  onsubmit     You re directly reading the onsubmit attribute s value  which must be a string   Instead  you should access the onsubmit property of the node      a button   parents  form   prop  onsubmit     Here s a quick test    lt form id  form1  action  foo1 htm  onsubmit  return valid    gt  lt  form gt   lt script gt  window onload   function          var form1   document getElementById  form1         function log s            document write   lt div gt     s     lt  div gt                function info v            return       typeof v          v             log  form1 onsubmit property      info form1 onsubmit        log  form1 onsubmit attribute      info form1 getAttribute  onsubmit          lt  script gt     This yields    form1 onsubmit property   function  function onsubmit event    return valid      form1 onsubmit attribute   string  return valid

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What browser are you using   alert typeof document getElementById  myform   onsubmit     This gives me  function  in IE7 and FireFox

[javascript] Testing if value is a function

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