How to tell if a JavaScript function is defined

Question

How do you tell if a function in JavaScript is defined   I want to do something like this  function something cool text  callback        alert text       if  callback    null   callback        But it gets me a     callback is not a function   error when callback is not defined

User · Answer

Most if not all previous answers have side effects to invoke the function

here best practice

you have function

_x000D_

function myFunction() {_x000D_
        var x=1;_x000D_
    }

_x000D_

_x000D_ direct way to test for it

_x000D_

//direct way_x000D_
        if( (typeof window.myFunction)=='function')_x000D_
            alert('myFunction is function')_x000D_
        else_x000D_
            alert('myFunction is not defined');

_x000D_

_x000D_ using a string so you can have only one place to define function name

_x000D_

//byString_x000D_
        var strFunctionName='myFunction'_x000D_
        if( (typeof window[strFunctionName])=='function')_x000D_
            alert(s+' is function');_x000D_
        else_x000D_
            alert(s+' is not defined');

_x000D_

User · Answer

If you look at the source of the library  Venkat Sudheer Reddy Aedama mentioned  underscorejs  you can see this     isFunction   function obj      return typeof obj     function     false       This is just my HINT  HINT answer

User · Answer

If you use http   underscorejs org  you have  http   underscorejs org  isFunction    isFunction callback

User · Answer

One-line solution   function something cool text  callback       callback  amp  amp  callback

User · Answer

I was looking for how to check if a jQuery function was defined and I didn t find it easily   Perhaps might need it     if typeof jQuery fn datepicker      undefined

User · Answer

Those methods to tell if a function is implemented also fail if variable is not defined so we are using something more powerful that supports receiving an string   function isFunctionDefined functionName        if eval  typeof     functionName         typeof Function               return true           if  isFunctionDefined  myFunction          myFunction foo

User · Answer

typeof callback      function

User · Answer

if  callback  amp  amp  typeof callback      function     Note that callback  by itself  evaluates to false if it is undefined  null  0  or false  Comparing to null is overly specific

User · Answer

typeof callback      function

User · Answer

Try   if    typeof callback    undefined

User · Answer

For global functions you can use this one instead of eval suggested in one of the answers   var global    function         return this         if  typeof global f      function       global f   function f1 shim                commonly used by polyfill libs          You can use global f instanceof Function as well  but afaik  the value of the Function will be different in different frames  so it will work only with a single frame application properly  That s why we usually use typeof instead  Note that in some environments there can be anomalies with typeof f too  e g  by MSIE 6-8 some of the functions for example alert had  object  type   By local functions you can use the one in the accepted answer  You can test whether the function is local or global too   if  typeof f      function       if  global f     f          console log  f is a global function        else         console log  f is a local function      To answer the question  the example code is working for me without error in latest browers  so I am not sure what was the problem with it   function something cool text  callback        alert text       if  callback    null   callback        Note  I would use callback     undefined instead of callback    null  but they do almost the same

User · Answer

typeof callback      function

User · Answer

Try   if  typeof callback      function

User · Answer

If you wish to redefine functions  it is best to use function variables  which are defined in their order of occurrence  since functions are defined globally  no matter where they occur   Example of creating a new function that calls a previous function of the same name   A function            first definition     if  typeof A    function      oldA A  A function       oldA          new definition

User · Answer

if   function      typeof callback

User · Answer

function something cool text  callback       alert text       if typeof callback    function             callback

User · Answer

Those methods to tell if a function is implemented also fail if variable is not defined so we are using something more powerful that supports receiving an string   function isFunctionDefined functionName        if eval  typeof     functionName         typeof Function               return true           if  isFunctionDefined  myFunction          myFunction foo

User · Answer

I might do  try      callback     catch e       I know there s an accepted answer  but no one suggested this  I m not really sure if this fits the description of idiomatic  but it works for all cases   In newer JavaScript engines a finally can be used instead

User · Answer

function something cool text  callback       alert text       if typeof callback    function             callback

User · Answer

If the callback   you are calling not just for one time in a function  you could initialize the argument for reuse   callback    typeof callback      function     callback   function        For example   function something cool text  callback           Initialize arguments     callback    typeof callback      function     callback   function           alert text        if  text    waitAnotherAJAX             anotherAJAX callback         else           callback              The limitation is that it will always execute the callback argument although it s undefined

User · Answer

Try   if    typeof callback    undefined

User · Answer

New to JavaScript I am not sure if the behaviour has changed but the solution given by Jason Bunting  6 years ago  won t work if possibleFunction is not defined    function isFunction possibleFunction      return  typeof possibleFunction     typeof Function        This will throw a ReferenceError  possibleFunction is not defined error as the engine tries to resolve the symbol possibleFunction  as mentioned in the comments to Jason s answer   To avoid this behaviour you can only pass the name of the function you want to check if it exists  So  var possibleFunction   possibleFunction        if   isFunction possibleFunction   return false    This sets a variable to be either the function you want to check or the empty object if it is not defined and so avoids the issues mentioned above

User · Answer

typeof callback      function

User · Answer

Try this   callback instanceof Function

User · Answer

Try   if    typeof callback    undefined

User · Answer

typeof callback      function

User · Answer

All of the current answers use a literal string  which I prefer to not have in my code if possible - this does not  and provides valuable semantic meaning  to boot    function isFunction possibleFunction      return typeof possibleFunction      typeof Function       Personally  I try to reduce the number of strings hanging around in my code       Also  while I am aware that typeof is an operator and not a function  there is little harm in using syntax that makes it appear as the latter

User · Answer

Most if not all previous answers have side effects to invoke the function

here best practice

you have function

_x000D_

function myFunction() {_x000D_
        var x=1;_x000D_
    }

_x000D_

_x000D_ direct way to test for it

_x000D_

//direct way_x000D_
        if( (typeof window.myFunction)=='function')_x000D_
            alert('myFunction is function')_x000D_
        else_x000D_
            alert('myFunction is not defined');

_x000D_

_x000D_ using a string so you can have only one place to define function name

_x000D_

//byString_x000D_
        var strFunctionName='myFunction'_x000D_
        if( (typeof window[strFunctionName])=='function')_x000D_
            alert(s+' is function');_x000D_
        else_x000D_
            alert(s+' is not defined');

_x000D_

User · Answer

Try   if    typeof callback    undefined

User · Answer

For global functions you can use this one instead of eval suggested in one of the answers   var global    function         return this         if  typeof global f      function       global f   function f1 shim                commonly used by polyfill libs          You can use global f instanceof Function as well  but afaik  the value of the Function will be different in different frames  so it will work only with a single frame application properly  That s why we usually use typeof instead  Note that in some environments there can be anomalies with typeof f too  e g  by MSIE 6-8 some of the functions for example alert had  object  type   By local functions you can use the one in the accepted answer  You can test whether the function is local or global too   if  typeof f      function       if  global f     f          console log  f is a global function        else         console log  f is a local function      To answer the question  the example code is working for me without error in latest browers  so I am not sure what was the problem with it   function something cool text  callback        alert text       if  callback    null   callback        Note  I would use callback     undefined instead of callback    null  but they do almost the same

User · Answer

One-line solution   function something cool text  callback       callback  amp  amp  callback

User · Answer

Try this   callback instanceof Function

User · Answer

Try   if  typeof callback      function

User · Answer

typeof callback      function

User · Answer

I might do  try      callback     catch e       I know there s an accepted answer  but no one suggested this  I m not really sure if this fits the description of idiomatic  but it works for all cases   In newer JavaScript engines a finally can be used instead

User · Answer

I was looking for how to check if a jQuery function was defined and I didn t find it easily   Perhaps might need it     if typeof jQuery fn datepicker      undefined

User · Answer

Try   if  typeof callback      function

User · Answer

This worked for me  if  cb  amp  amp  typeof  eval  cb          function         eval  cb

User · Answer

if  callback  amp  amp  typeof callback      function     Note that callback  by itself  evaluates to false if it is undefined  null  0  or false  Comparing to null is overly specific

User · Answer

This worked for me  if  cb  amp  amp  typeof  eval  cb          function         eval  cb

User · Answer

If you look at the source of the library  Venkat Sudheer Reddy Aedama mentioned  underscorejs  you can see this     isFunction   function obj      return typeof obj     function     false       This is just my HINT  HINT answer

User · Answer

New to JavaScript I am not sure if the behaviour has changed but the solution given by Jason Bunting  6 years ago  won t work if possibleFunction is not defined    function isFunction possibleFunction      return  typeof possibleFunction     typeof Function        This will throw a ReferenceError  possibleFunction is not defined error as the engine tries to resolve the symbol possibleFunction  as mentioned in the comments to Jason s answer   To avoid this behaviour you can only pass the name of the function you want to check if it exists  So  var possibleFunction   possibleFunction        if   isFunction possibleFunction   return false    This sets a variable to be either the function you want to check or the empty object if it is not defined and so avoids the issues mentioned above

User · Answer

typeof callback      function

User · Answer

All of the current answers use a literal string  which I prefer to not have in my code if possible - this does not  and provides valuable semantic meaning  to boot    function isFunction possibleFunction      return typeof possibleFunction      typeof Function       Personally  I try to reduce the number of strings hanging around in my code       Also  while I am aware that typeof is an operator and not a function  there is little harm in using syntax that makes it appear as the latter

User · Answer

if  callback  amp  amp  typeof callback      function     Note that callback  by itself  evaluates to false if it is undefined  null  0  or false  Comparing to null is overly specific

User · Answer

if   function      typeof callback

User · Answer

All of the current answers use a literal string  which I prefer to not have in my code if possible - this does not  and provides valuable semantic meaning  to boot    function isFunction possibleFunction      return typeof possibleFunction      typeof Function       Personally  I try to reduce the number of strings hanging around in my code       Also  while I am aware that typeof is an operator and not a function  there is little harm in using syntax that makes it appear as the latter

User · Answer

If you use http   underscorejs org  you have  http   underscorejs org  isFunction    isFunction callback

User · Answer

All of the current answers use a literal string  which I prefer to not have in my code if possible - this does not  and provides valuable semantic meaning  to boot    function isFunction possibleFunction      return typeof possibleFunction      typeof Function       Personally  I try to reduce the number of strings hanging around in my code       Also  while I am aware that typeof is an operator and not a function  there is little harm in using syntax that makes it appear as the latter

User · Answer

function something cool text  callback       alert text       if typeof callback    function             callback

User · Answer

Try   if  typeof callback      function

User · Answer

If you wish to redefine functions  it is best to use function variables  which are defined in their order of occurrence  since functions are defined globally  no matter where they occur   Example of creating a new function that calls a previous function of the same name   A function            first definition     if  typeof A    function      oldA A  A function       oldA          new definition

User · Answer

typeof callback      function

User · Answer

if   function      typeof callback

User · Answer

If the callback   you are calling not just for one time in a function  you could initialize the argument for reuse   callback    typeof callback      function     callback   function        For example   function something cool text  callback           Initialize arguments     callback    typeof callback      function     callback   function           alert text        if  text    waitAnotherAJAX             anotherAJAX callback         else           callback              The limitation is that it will always execute the callback argument although it s undefined

User · Answer

if  callback  amp  amp  typeof callback      function     Note that callback  by itself  evaluates to false if it is undefined  null  0  or false  Comparing to null is overly specific

User · Answer

function something cool text  callback       alert text       if typeof callback    function             callback

User · Answer

if   function      typeof callback

[javascript] How to tell if a JavaScript function is defined

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