Call a function only if it is defined

exec.js

var exec = (function(global) {
	return function(name, args, context) {
		var func = (context = context || global)[name];
		return (typeof func != 'undefined') && func.apply(context, args);
	};
})(this);

// Use like:
var timesTwo = function(x) {
    return x * 2;
};

exec('timesTwo', [2]); // => 4
exec('nonExistantFunc'); // => false