Duck Typing in Javascript

The rule of “Duck Typing” is

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

In a class-based object-oriented programming language (C++, for example) to make both objects look like a duck you must inherit their classes from a common “interface” class, so the compiler would let you call duck methods on them. That is called a strong typing.

Now this is how it’s done in Javascript:

var duck = {  
    appearance: "feathers",  
    quack: function duck_quack(what) {  
        print(what + " quack-quack!");  
    },  
    color: "black"  
};

var someAnimal = {  
    appearance: "feathers",  
    quack: function animal_quack(what) {  
        print(what + " whoof-whoof!");  
    },  
    eyes: "yellow"  
};  

function check(who) {  
    if ((who.appearance == "feathers") && (typeof who.quack == "function")) {  
        who.quack("I look like a duck!\n");  
        return true;  
    }  
    return false;  
}  

check(duck);  // true
check(someAnimal);  // true

See, the check function check whether the passed object looks like a duck (it checks appearance and its’ ability to quack). We pass two different objects to it and it will return true on both. Besides the appearance and quacking these may be completely different things, but IN THIS PARTICULAR check function they behave the same way (have a common interface), they both look like a “duck”. We can call the quack method on both objects (and who cares what they really are).