CODEWITHSUNDEEP
javascriptprototype
Tim
Tim

Reputation: 340

Confused about prototype in javascript

var y=function(){
    return new y.prototype.greeting();
}
y.prototype={
    greeting:function(){
        alert("hello world");
    }
}

new y();

the above code will alert("hello world");

but if I remove the .prototype and change that line to return new y.greeting();

an error will be occurred:

undefined is not a function

var y=function(){
    return new y.greeting();
}
y.prototype={
    greeting:function(){
        alert("hello world");
    }
}

new y();

why I can't call the greeting method without prototype here?

thanks a lot

Upvotes: 0

Views: 66

Answers (2)

Sean Vieira
Sean Vieira

Reputation: 159855

y is a function, the [[Prototype]] of which is Function - so when you ask the interpreter to look up y.greeting for you, it first looks at y itself, then it checks Function.prototype and then it checks Object.prototype.

When you create a new y then the prototype property of y will be set on the object that is created. So if you did new (new y()).greetings() then you would get your alert.

Another way to think about it is the prototype property of a constructor function is the prototype of any child objects that will be created by calling new constructor. The actual internal [[Prototype]] of the constructor will always be based on whatever constructed it.

You can see this in the example I put together below. Object.getPrototypeOf will return the internal [[Prototype]] property, so we can see what is actually going on:

> var test = function() {}
> Object.getPrototypeOf(test)
function Empty() {}

// Setting the prototype property
// does not change the [[Prototype]] of test
> test.prototype = {x: 1}
> Object.getPrototypeOf(test)
function Empty() {}

// But it *does* change the prototype of
// objects *created by* test.
> var x = new test
> Object.getPrototypeOf(x)
Object {x: 1}

Upvotes: 2

Sayed
Sayed

Reputation: 631

return new y.greeting(); tries to access an attribute (function) of y which doesn't really exist. That's why it throws the error 'undefined is not a function' because, the attribute of y contains a variable which contains a function. It's like three floors, where you cannot go to ground floor from the second floor without going to first floor; kind of hierarchy. Got it?

Upvotes: 0

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