How does JavaScript parse chained ternary expressions?

Question

I'm reading through the jQuery source code for jQuery.filter and I stumbled upon this heaping pile of

jQuery.filter = function( expr, elems, not ) {
    var elem = elems[ 0 ];

    if ( not ) {
        expr = ":not(" + expr + ")";
    }

    return elems.length === 1 && elem.nodeType === 1 ?
        jQuery.find.matchesSelector( elem, expr ) ? [ elem ] : [] :
        jQuery.find.matches( expr, jQuery.grep( elems, function( elem ) {
            return elem.nodeType === 1;
        }));
};

So in short we have

return "a" && "b" ? "c" ? "d" : "e" : "f";

where each string could be a varying value

My question isn't how to decipher this code, but my brain is tying in knots trying to evaluate the logic being used here.

Can anyone help me understand how JavaScript evaluates this return expression?

It seems evident that c ? d : e must be a sub-expression (there's no other meaningful way to interpret those tokens). Therefore it really comes down to whether a && b ? x : y is (a && b) ? x : y or a && (b ? x : y). But surely this is solved with a trivial Google search for "javascript precedence" or similar? — Oliver Charlesworth
– Oliver Charlesworth, Commented Sep 17, 2014 at 23:31
Someone in jQuery should learn to use braces though it cost 2 bits in the framework size... — gdoron
– gdoron, Commented Sep 17, 2014 at 23:31

p.s.w.g · Accepted Answer · 2014-09-17 23:35:30Z

4

The conditional operator is right-associative and the logical operators have higher precedence, so this:

return "a" && "b" ? "c" ? "d" : "e" : "f";

Is equivalent to this:

return ( ("a" && "b") ? ("c" ? "d" : "e") : "f" );

Or in full:

if ("a" && "b") {
    if ("c") {
        return "d";
    } else {
        return "e";
    }
} else {
    return "f";
}

Reference

MDN: Operator precedence

answered Sep 17, 2014 at 23:35

p.s.w.g

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1 Comment

Mulan Over a year ago

Thanks, pony. You're a real pretty pony.

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