CODEWITHSUNDEEP
javascript
Kevin Meredith
Kevin Meredith

Reputation: 41909

JS `this` on Function Invocation

Working through Secrets of the JavaScript Ninja, I'm looking at constructors with respect to the this keyword.

function Ninja() { 
   this.skulk = function() { return this; }
}

Example:

var ninja1 = new Ninja();

window.onload = function() {
    assert(ninja1.skulk() === ninja1,
        "the 1st ninja is skulking");
};

Output: the 1st ninja is skulking

However, if I add var windowNinja = Ninja(), why am I seeing this JavaScript error in Chrome?

    assert(windowNinja.skulk() === window,
        "the window ninja is skulking");

output: JavaScript error: Uncaught TypeError: Cannot call method 'skulk' of undefined

Upvotes: 1

Views: 107

Answers (2)

Musa
Musa

Reputation: 97672

Ninja doesn't return anything so if you dont initiate an instance of it with new it returns undefined and that is stored in windowNinja. You can just call 

Ninja();
assert(window.skulk() === window,
    "the window ninja is skulking");

So the this in Ninja is the global object(window) so you're assigning a method skulk to it. which returns the object that invoked it.

Alternately you can add a return to the function

function Ninja() { 
   this.skulk = function() { return this; }
   return  this;
}

Upvotes: 2

0xFADE
0xFADE

Reputation: 832

You left out new, var windowNinja = new Ninja();

Upvotes: 0

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