Javascript: || instead of IF statement - is this legal and cross browser valid?

Question

It seems that:

if (typeof a == 'undefined') {     a = 0; } 

and

(typeof a != 'undefined') || (a = 0) 

has the same effect in Javascript.

I really like the second one because it is short, one line code, but is this legal, and cross browser valid? I mean, jslint says it has errors. Should I use it without concerns?

Answer 1

IMHO || (a = 0) is way too similar to || (a == 0) and thus confusing. One day overzealous developer will just "fix it", changing the meaning of your code. And every other developer will have to sit for a while to figure out whether this was your intent or just a simple bug.

And this is in fact what JSLint is trying to say:

Expected a conditional expression and instead saw an assignment.

I avoid using confusing constructs as they hurt readability. a = a || 0; is way more recognizable and similar in meaning.

Answer 2

Why not something more simple, like:

a = a || 0; 

or

a = a ? a : 0; 

In both of these cases, you can also clearly see that something is being assigned to a, right at the start of the line, without resorting to reading the whole thing, and figuring out if there are any game-changing function-calls happening on either the left or right side... or figuring out what both sides do, in general, to decide how many potential program-wide changes there might be.

If you need to include the whole type-check, it's still not that large.

a = (typeof a !== "undefined") ? a : 0;  // [parentheses are there for clarity]