Question

I have done a fair amount of programming in JavaScript/JQuery.

But I never used “prototype”. In fact, I have no clue what it means.

Do you have a practical example when it is useful?

Answer 1

Simplest example:

function Foo() {
    this.bar = function() {
        return 42;
    }
}

Now you can create as many instances of Foo as you want and call bar():

var a = new Foo();
var b = new Foo();

Even though both object have bar() method which is exactly the same in both cases, these are distinct methods. In fact, each new Foo object will have a new copy of this method.

On the other hand:

function Foo() {}
Foo.prototype.bar = function() {
    return 42;
}    

has the same end result but the function is stored only once in object prototype rather than in an object itself. This may be a deal breaker if you create tons of Foo instances and want to save some memory.