How to check if function exists in JavaScript

Question

My code is  function getID  swfID         if navigator appName indexOf  Microsoft      -1             me   window swfID         else            me   document swfID             function js to as  str         me onChange str       However  sometimes my onChange does not load  Firebug errors with     me onChange is not a function   I want to degrade gracefully because this is not the most important feature in my program  typeof gives the same error   Any suggestions on how to make sure that it exists and then only execute onChange    None of the methods below except try catch one work

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And then there is this       document exitPointerLock    Function

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If you re checking for a function that is a jQuery plugin  you need to use   fn myfunction  if  typeof   fn mask      function             zip   mask  00000

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How about   if  functionName  in Obj         code     e g    var color1   new String  green     length  in color1    returns true  indexOf  in color1    returns true  blablabla  in color1    returns false   or as for your case   if  onChange  in me         code     See MDN docs

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Simple function that will tell if the function is defined or not function is function func        return typeof window func       undefined   amp  amp    isFunction window func         usage  if  is function  myFunction             alert  myFunction defined          else           alert  myFunction not defined

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If you re using eval to convert a string to function  and you want to check if this eval d method exists  you ll want to use typeof and your function string inside an eval   var functionString    nonexsitantFunction  eval  typeof     functionString     returns  undefined  or  function    Don t reverse this and try a typeof on eval  If you do a ReferenceError will be thrown   var functionString    nonexsitantFunction  typeof eval functionString      returns ReferenceError   function  is not defined

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Try typeof -- Look for  undefined  to say it doesn t exist   function  for a function  JSFiddle for this code  function thisishere         return false    alert  thisishere   is a     typeof thisishere   alert  thisisnthere   is     typeof thisisnthere     Or as an if   if  typeof thisishere      function            function exists     Or with a return value  on a single line   var exists    typeof thisishere      function      Value if true     Value if false   var exists    typeof thisishere      function      Returns true or false

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Try something like this   if  typeof me onChange      undefined             safe to use the function     or better yet  as per UpTheCreek upvoted comment   if  typeof me onChange      function             safe to use the function

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The Underscore js library defines it in the isFunction method as this  which comments suggest may cater for some browser bugs   typeof obj     function     false   http   underscorejs org docs underscore html section-143

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I had the case where the name of the function varied according to a variable  var  x  in this case  added to the functions name  This works   if   typeof window  afunction   x       function      window  afunction   x

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I would suspect that me is not getting correctly assigned onload   Moving the get ID call into the onclick event should take care of it   Obviously you can further trap as previously mentioned   function js to as  str      var me   get ID  jsExample      if  me  amp  amp  me onChange        me onChange str

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just pass your tested function name instead of myFunctionName if     isFunction   fn myFunctionName          console log   write your code here

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Try this one   Window function exists function function name scope     Setting default scope of none is provided If typeof scope      undefined   scope window    Checking if function name is defined If  typeof function name      undefined   throw new  Error  You have to provide an valid function name       The type container var fn   typeof scope function name      Function type If fn      function   return true    Function object type if fn indexOf  function      false  return true   return false      Be aware that I ve write this with my cellphone Might contain some uppercase issues and or other corrections needed like for example functions name  If you want a function like PHP to check if the var is set   Window isset function  variable con   If typeof variable con      undefined   return true  return false

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I have also been looking for an elegant solution to this problem  After much reflection  I found this approach best   const func    me onChange     str   gt       func str

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I always check like this   if  myFunction  return false     just place it before any code that uses this function

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This simple jQuery code should do the trick   if  jQuery isFunction functionName         functionName

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I have tried the accepted answer  however   console log typeof me onChange     returns  undefined   I ve noticed that the specification states an event called  onchange  instead of  onChange   notice the camelCase    Changing the original accepted answer to the following worked for me   if  typeof me onchange      function           safe to use the function

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I like using this method   function isFunction functionToCheck      var getType         return functionToCheck  amp  amp  getType toString call functionToCheck        object Function        Usage   if   isFunction me onChange          me onChange str      call the function with params

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function js to as  str         if  me  amp  amp  me onChange           me onChange str

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Here is a working and simple solution for checking existence of a function and triggering that function dynamically by another function   Trigger function  function runDynamicFunction functionname         if  typeof window functionname      function       check availability          window functionname   this is from the function it       run function and pass a parameter to it           and you can now generate the function dynamically maybe using php like this  function runThis func my Parameter        alert my Parameter    triggerd        now you can call the function using dynamically generated event    lt  php   name frm somware   runThis func    echo   lt input type  button  value  Button  onclick  runDynamicFunction      name frm somware        gt       gt    the exact HTML code you need is   lt input type  button  value  Button  onclick  runDynamicFunction  runThis func     gt

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I would suggest using  function hasMethod subject  methodName      return subject    null  amp  amp  typeof subject methodName      quot function quot      The first check subject    null filters out nullish values  null and undefined  which don t have any properties  Without this check subject methodName  could throw an error   TypeError   undefined null  has no properties  Checking for only a truthy value isn t enough  since 0 and  quot  quot  are both falsy but do have properties  After validating that subject is not nullish you can safely access the property and check if it matches typeof subject methodName      quot function quot    Applying this to your code you can now do  if  hasMethod me   quot onChange quot        me onChange str

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function sum nb1 nb2           return nb1 nb2             try         if sum      undefined    test if the function is defined before call it            sum 3 5                   once the function is exist you can call it                   catch e          console log  function not defined     the function is not defined or does not exists

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To illustrate the preceding answers  here a quick JSFiddle snippet      x000D   x000D  function test      x000D  console log   x000D   x000D    x000D   x000D  console log typeof test      gt  gt   function  x000D   x000D     implicit test  in javascript if an entity exist it returns implcitly true unless the element value is false as   x000D     var test   false x000D  if test   console log true   x000D  else console log false   x000D   x000D     test by the typeof method x000D  if  typeof test      function    console log true   x000D  else console log false   x000D   x000D   x000D     confirm that the test is effective    x000D     - entity with false value x000D  var test2   false x000D  if test2   console log true   x000D  else console log false   x000D   x000D     confirm that the test is effective   x000D     - typeof entity x000D  if  typeof test     foo    console log true   x000D  else console log false   x000D   x000D     Expected   x000D  function x000D  true  x000D  true  x000D  false x000D  false x000D     x000D   x000D   x000D

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In a few words  catch the exception    I am really surprised nobody answered or commented about Exception Catch on this post yet   Detail  Here goes an example where I try to match a function which is prefixed by mask  and suffixed by the form field  name   When JavaScript does not find the function  it should throw an ReferenceError which you can handle as you wish on the catch section    x000D   x000D  function inputMask input    x000D    try   x000D      let maskedInput   eval  mask   input name   x000D   x000D      if typeof maskedInput      undefined   x000D          return input value  x000D      else x000D          return eval  mask   input name  input   x000D   x000D      catch e    x000D      if  e instanceof ReferenceError    x000D        return input value  x000D        x000D      x000D    x000D   x000D   x000D

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Modern Javascript to the rescue  In 2021  this is solved  at the language level in Javascript  and Typescript too  with the new Optional Chaining syntax me onChange   str   It s as simple as that - onChange only gets called if it exists  If onChange doesn t exist  nothing happens  and the expression returns undefined  So if onChange   would ordinarily return a value  you d want to check value     undefined before continuing  An extra thing to note - if onChange does exist but is not a function  this will throw a TypeError  This is as you would expect  it s the same behaviour as calling any non-function as a function  just worth pointing out that Optional Chaining doesn t do any magic to make this go away    Optional Chaining is a stage 4 TC39 proposal  so isn t technically in the ECMAScript spec yet  But  stage 4 means it s finalised and essentially guaranteed to be included in the next version  You can use Optional Chaining today via Babel or Typescript with confidence it won t change

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Put double exclamation mark i e    before the function name that you want to check  If it exists  it will return true   function abc        window abc     return true   window abcd     return false

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For me the easiest way    function func exists fname      return  typeof window fname       function

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I ll go 1 step further to make sure the property is indeed a function  function js to as  str         if  me  amp  amp  me onChange  amp  amp  typeof me onChange      function              me onChange str

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Didn t see this suggested      me onChange  amp  amp  me onChange str    Basically if me onChange is undefined  which it will be if it hasn t been initiated  then it won t execute the latter part  If me onChange is a function  it will execute me onChange str    You can even go further and do   me  amp  amp  me onChange  amp  amp  me onChange str     in case me is async as well

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With no conditions  me onChange function       function getID  swfID         if navigator appName indexOf  Microsoft      -1             me   window swfID         else            me   document swfID             function js to as  str         me onChange str

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I had this problem   if  obj  amp  amp  typeof obj      function             kept throwing a reference error if obj happened to be undefined   In the end I did the following   if  typeof obj      undefined   amp  amp  typeof obj      function             A colleague pointed out to me that checking if it s      undefined  and then      function  is redundant of course   Simpler   if  typeof obj      function             Much cleaner and works great

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function function exists function name        return eval  typeof     function name       function     alert function exists  test     alert function exists  function exists       OR  function function exists func name          discuss at  http   phpjs org functions function exists       original by  Kevin van Zonneveld  http   kevin vanzonneveld net       improved by  Steve Clay      improved by  Legaev Andrey      improved by  Brett Zamir  http   brett-zamir me         example 1  function exists  isFinite           returns 1  true    if  typeof func name      string         func name   this window func name         return typeof func name      function

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This will verify if the function exists  if so it will be executed  me onChange  amp  amp  me onChange str     Thus the error TypeError  me onChange is not a function is prevent

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function isFunction  o     return null     o  amp  amp   quot function quot      typeof o  amp  amp    o apply

[javascript] How to check if function exists in JavaScript?

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