Check if a variable is of function type

Solution 1:

if (typeof v === 'function') {
    // do something
}

Solution 2:

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 && {}.toString.call(functionToCheck) === '[object Function]';
}

Note: This solution doesn't work for async functions, generators or proxied functions. Please see other answers for more up to date solutions.

Solution 3:

There are several ways so I will summarize them all

1. 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.
2. 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
3. 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

Solution 4:

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