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

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.