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.