Since undeclared assignments are such a misleading part of the language, ECMAScript 5 — newest version of ECMAScript — specifies that when they occur in a strict mode, a ReferenceError should be thrown. This makes for more robust code with less chance for unexpected errors. And since strict mode is more or less a future direction of the language, it makes sense to avoid undeclared assignments as if they never existed.

http://perfectionkills.com/unnecessarily-comprehensive-look-into-a-rather-insignificant-issue-of-global-objects-creation/

I haven’t been able to find any information on how IE gets the second form wrong.

Personally, I’ve been using:

var fooObj;
if (fooObj === undefined) {
fooObj = {};
}


this.pity || (this.pity = {});

the reason is it avoids an extra “same value” assignment if that global already exist and I do the same in functions for passed parameters:

function doSomething (a) {
a || (a = “defaultValue”);
}

good to spread the voice whatever the correct…

One of the most useful cases for me would be as a default value for function parameters. Consider:

function doSomething (a) {
a = a || ‘defaultValue’;
}

If the variable ‘a’ is undefined here, it will get a default value instead, making the variable ‘a’ useful to the rest of the function.

Though if you prefer, I guess it could be written as:

function doSomething (a) {
a = (a !== undefined ? a : ‘defaultValue’);
}

If you assume that the web developer audiences that program in ignorance of the language and the readership of this blog (or the readership of any JavaScript blog author who adopts this method) are mutually exclusive, then one might trend towards agreeing your third form is appropriate.

But, seeing that most developers “learn” from copy and paste, it would seem that the second form would be safer, if for no other reason than to prevent those abominable globals we all abhor.