Check if a variable is of function type

Question

Suppose I have any variable  which is defined as follows   var a   function       Statements        I want a function which checks if the type of the variable is function-like  i e     function foo v   if  v is function type       do something       foo a     How can I check if the variable a is of type Function in the way defined above

User · Answer

you should use typeOf operator in js   var a function        alert  fun a      alert typeof a     alerts  function

User · Answer

Try the instanceof operator  it seems that all functions inherit from the Function class      Test data var f1   function      alert  test      var o1     Name   Object 1     F est   function         var o2   new F est        Results alert f1 instanceof Function      true alert o1 instanceof Function      false alert o2 instanceof Function      false

User · Answer

x000D   x000D  var foo   function      x000D  if  typeof foo      function     x000D    alert  is function   x000D    x000D   x000D   x000D

User · Answer

I found that when testing native browser functions in IE8  using toString  instanceof  and typeof did not work  Here is a method that works fine in IE8  as far as I know    function isFn f       return    f  amp  amp  f call  amp  amp  f apply       Returns true in IE7 8 isFn document getElementById     Alternatively  you can check for native functions using    getElementById  in document   Though  I have read somewhere that this will not always work in IE7 and below

User · Answer

The below seems to work for me as well  tested from node js    var isFunction   function o         return Function prototype isPrototypeOf o       console log isFunction function           true console log isFunction          false

User · Answer

For those who s interested in functional style  or looks for more expressive approach to utilize in meta programming  such as type checking   it could be interesting to see Ramda library to accomplish such task   Next code contains only pure and pointfree functions   const R   require  ramda     const isPrototypeEquals   R pipe Object getPrototypeOf  R equals    const equalsSyncFunction   isPrototypeEquals      gt        const isSyncFunction   R pipe Object getPrototypeOf  equalsSyncFunction     As of ES2017  async functions are available  so we can check against them as well   const equalsAsyncFunction   isPrototypeEquals async      gt        const isAsyncFunction   R pipe Object getPrototypeOf  equalsAsyncFunction     And then combine them together   const isFunction   R either isSyncFunction  isAsyncFunction     Of course  function should be protected against null and undefined values  so to make it  safe    const safeIsFunction   R unless R isNil  isFunction     And  complete snippet to sum up   const R   require  ramda     const isPrototypeEquals   R pipe Object getPrototypeOf  R equals    const equalsSyncFunction   isPrototypeEquals      gt       const equalsAsyncFunction   isPrototypeEquals async      gt        const isSyncFunction   R pipe Object getPrototypeOf  equalsSyncFunction   const isAsyncFunction   R pipe Object getPrototypeOf  equalsAsyncFunction    const isFunction   R either isSyncFunction  isAsyncFunction    const safeIsFunction   R unless R isNil  isFunction       ---  console log safeIsFunction  function           console log safeIsFunction       gt         console log safeIsFunction   async      gt          console log safeIsFunction  new class        console log safeIsFunction         console log safeIsFunction         console log safeIsFunction   a      console log safeIsFunction  1     console log safeIsFunction  null     console log safeIsFunction  undefined       However  note the this solution could show less performance than other available options due to extensive usage of higher-order functions

User · Answer

I think you can just define a flag on the Function prototype and check if the instance you want to test inherited that  define a flag   Function prototype isFunction   true     and then check if it exist   var foo   function      foo isFunction     will return true   The downside is that another prototype can define the same flag and then it s worthless  but if you have full control over the included modules it is the easiest way

User · Answer

There are several ways so I will summarize them all   Best way is     function foo v   if  v instanceof Function      do something          Most performant  no string comparison  and elegant solution - the instanceof operator has been supported in browsers for a very long time  so don t worry - it will work in IE 6    Next best way is    function foo v   if  typeof v      function       do something          disadvantage of typeof is that it is susceptible to silent failure  bad  so if you have a typo  e g   finction   - in this case the  if  will just return false and you won t know you have an error until later in your code   The next best way is     function isFunction functionToCheck        var getType           return functionToCheck    getType toString call functionToCheck        object Function        This has no advantage over solution  1 or  2 but is a lot less readable  An improved version of this is    function isFunction x        return Object prototype toString call x       object Function         but still lot less semantic than solution  1

User · Answer

jQuery  deprecated since version 3 3  Reference    isFunction functionName     AngularJS Reference  angular isFunction value     Lodash Reference    isFunction value     Underscore Reference    isFunction object      Node js deprecated since v4 0 0 Reference  var util   require  util    util isFunction object

User · Answer

If you use Lodash you can do it with   isFunction     isFunction function            gt  true    isFunction  abc         gt  false    isFunction true        gt  false    isFunction null        gt  false   This method returns true if value is a function  else false

User · Answer

The solution as some previous answers has shown is to use typeof  the following is a code snippet In NodeJs       function startx           console log  startx function called             var fct       fct   startx     startx   if  typeof fct  startx       function       check if function then execute it     fct  startx

User · Answer

grandecomplex  There s a fair amount of verbosity to your solution  It would be much clearer if written like this   function isFunction x      return Object prototype toString call x       object Function

User · Answer

Underscore js uses a more elaborate but highly performant test     isFunction   function obj      return    obj  amp  amp  obj constructor  amp  amp  obj call  amp  amp  obj apply        See  http   jsperf com alternative-isfunction-implementations  EDIT  updated tests suggest that typeof might be faster  see http   jsperf com alternative-isfunction-implementations 4

User · Answer

if  typeof v      function            do something

User · Answer

Something with more browser support and also include async functions could be   const isFunction   value   gt  value  amp  amp   Object prototype toString call value        object Function       function      typeof value    value instanceof Function     and then test it like   isFunction isFunction     true isFunction function         true isFunction     gt         true isFunction     gt   return 1      true isFunction async function asyncFunction         true isFunction Array     true isFunction Date     true isFunction Object     true isFunction Number     true isFunction String     true isFunction Symbol     true isFunction        false isFunction        false isFunction  function      false isFunction true     false isFunction 1     false isFunction  Alireza Dezfoolian      false

User · Answer

Sure underscore s way is more efficient  but the best way to check  when efficiency isn t an issue  is written on underscore s page linked by  Paul Rosania   Inspired by underscore  the final isFunction function is as follows   function isFunction functionToCheck     return functionToCheck  amp  amp     toString call functionToCheck        object Function

User · Answer

An other simply way   var fn   function       if  fn constructor     Function         true   else        false

User · Answer

Since node v0 11 you can use the standard util function    var util   require  util    util isFunction  foo

[javascript] Check if a variable is of function type

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