typeof yourFunction === 'function'

typeof String.link === 'function' // false
typeof String.link() === 'function' // Uncaught error ...

typeof String.prototype.link === 'function' //true

function foo(){
    alert('from foo');
}

foo.bar = function(){
    alert('bar on foo');
}

foo();     //from foo
foo.bar(); //bar on foo

String//Defines the Object
String.prototype//Defines the methods and properties that will be bound to a var of the type String
'foo'//This is a string
'foo'.link//So it has the link method
String//This is an Objecy that defines the strings
String.link//So it doesn't have the link method
String.prototype//An object that stores the methods and properties of the vars of type String
String.prototype.link//So it has the link method