CODEWITHSUNDEEP
javascript
C.J.
C.J.

Reputation: 6879

When creating a global object in JavaScript why do I need to specify the this keyword

Say I'm trying to create a global variable x if it doesn't already exist:

x = x || {};            //This fails. x is not defined
this.x = this.x || {};  //But this works

I'm running this in Firebug and was surprised that the 1st line failed. I expected x to be attached to the global window object, but it didn't work unless I specified the this.

I'm hoping to improve my understanding of the language. Can someone explain to me why this is?

Thanks for any help.

Upvotes: 2

Views: 47

Answers (1)

Matt
Matt

Reputation: 75327

  • If you try and use an undeclared variable a part of an expression, you'll get a ReferenceError thrown.

  • If you try and assign an undeclared variable, the variable will be declared as an implicit global. Implicit globals are bad.

x = x || {};  
    ^  its this x that breaks it.

To correctly check whether a variable is declared, you should use the typeof variable === "undefined" check.

Undefined attributes however (in your second example, x is an attribute on this), hold the value undefined by default.

Upvotes: 2

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