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

Answer 1

var foo = function(){};
if (typeof foo === "function") {
  alert("is function")
}

Answer 2

If you use Lodash you can do it with _.isFunction.

_.isFunction(function(){});
// => true

_.isFunction(/abc/);
// => false

_.isFunction(true);
// => false

_.isFunction(null);
// => false

This method returns true if value is a function, else false.

Answer 3

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);

Answer 4

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 && f.call && 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.

Answer 5

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]();
  }

Answer 6

Underscore.js uses a more elaborate but highly performant test:

_.isFunction = function(obj) {
  return !!(obj && obj.constructor && obj.call && 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