what does this mean? javascript question

Question

var a = window.a || {};

+1 I am going to guess most people don't understand your reference... http://www.youtube.com/watch?v=OQSNhk5ICTI — Nix, Oct 28 '10 at 13:14
Same thing as this - coalescing operator- http://stackoverflow.com/questions/446835/what-do-two-question-marks-together-mean-in-c — Phil, Oct 28 '10 at 13:18
@Phil: no, it's not quite. See revision history of http://stackoverflow.com/posts/61158/revisions for someone else who used to think the same. — Roatin Marth, Oct 28 '10 at 13:21
Similar concept though. It's operating as a coalescing operator but branches on a much wider range of values than just `null` — Phil, Oct 28 '10 at 13:28

Phil · Accepted Answer · 2010-10-28T13:19:22.420

7

It means a will be assigned window.a if it is not null or undefined, otherwise, it will equal an empty object

edited Oct 28 '10 at 13:19

answered Oct 28 '10 at 13:13

Phil

score 1 · Answer 2 · answered Oct 28 '10 at 14:05

To answer the unasked question: this is used to make sure "a" will be valid.

Without it, when calling a.someFieldHere you might get exception saying "a is undefined", with such code in place you won't get such error.

It's useful when "a" is created elsewhere in some other code that not always get executed.

Kind of insurance policy. :)

2 Answers2