Question

I’ve seen code like this on the web:

function MyEventHandler(e)
{
    var ev = e || event;
    var target = ev.srcElement || ev.target
}

In essence, the || operator is used as a shorthand for a?a:b. As far as I can tell – it works on all browsers. But bringing up specs for, say, JScript, I see:

Performs a logical disjunction on two expressions.

and

JScript uses the following rules for converting non-Boolean values to Boolean values:

• All objects are considered true.

So… according to this the result should be a boolean true/false. I’m just wondering – am I walking the knife-edge of undocumented behavior, or is there some fine implication here that I haven’t picked up?

Answer 1

That behavior is quite well documenten. See Chapter 11.11 of the ECMAScript Specification: http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf

Unfortunately it’s not very easy to understand, however the last paragraph states:

The value produced by a && or ||
operator is not necessarily of type
Boolean. The value produced will
always be the value of one of the two
operand expressions.