Question

I’m writing a JavaScript parser and while testing it on the prototype.js library, it threw up an error on this code:

line 4000:

while ( (chunker.exec(""), m = chunker.exec(soFar)) !== null ) {
    soFar = m[3];

    parts.push( m[1] );

    if ( m[2] ) {
        extra = m[3];
        break;
    }
}

I reduced it down to this:

while ( (a, b) == c ) {}

Is this really valid JavaScript? According to ECMA-262, a while loop has this syntax:

while ( Expression ) Statement

What sort of expression is (a, b) == c? I didn’t think tuples were supported in JavaScript?

Answer 1

That’s not a tuple; that’s the comma operator (also see Wikipedia). The comma operator evaluates the first operand and then the second operand and yields the value of the second.