javascript - Check if a variable is of function type

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

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]';
}