How can I check if a javascript variable is functi

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?

Answer 1

Since node v0.11 you can use the standard util function :

var util = require('util');
util.isFunction('foo');

Answer 2

An other simply way:

var fn = function () {}
if (fn.constructor === Function) {
  // true
} else {
  // false
}

Answer 3

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

Answer 4

you should use typeOf operator in js.

var a=function(){
    alert("fun a");
}
alert(typeof a);// alerts "function"

Answer 5

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);