Yes, it's valid, and it runs fine in Chrome:

var a, b, c;



a = 6;

b = 7;

c = a !== b ? (a = 1, b = 2) : (a = 2, b = 1);

console.log(a =  + a);

console.log(b =  + b);

console.log(c =  + c);

I'm not saying it's a remotely good idea in code humans are meant to read. :-) I expect jamietre is correct in the comments when he/she says it looks like the result of minification.

The comma operator is a binary operator (an operator accepting two operands). It evaluates its left-hand operand (thus causing any side-effects it has, such as assignment), throws that result away, then evalutes its right-hand operand (thus causing its side-effects if any) and takes that result as its result value. If you have multiple comma operators in a row, the overall expression is evaluated in order, left-to-right, with the final result being the value resulting from the right-most operand evaluation.

And of course, you know the conditional operator (a ternary operator — one accepting three operands) is used to pick one of two sub-expressions to evaluate, on the basis of an initial expression.

So that line is very...expressive...what with a total of seven* different expressions inside it.

So in that example, the result of the overall expression is 2 if a !== b initially, or 1 if a === b initially, with the side-effects of setting a and b.

It's the side effects that make it, in my view, a questionable choice. And of course, there's no reason to use the comma operator if the left-hand operand doesn't have side effects.

* Yes, seven of 'em packed into that overall ternary:

• a !== b


• the first comma expression


• a = 1


• b = 2


• the second comma expression


• a = 2


• b = 1

Re your edit with the actual statement, that one works too:

function test(a) {

    var b = 7,

        d = 1,

        e = 2,

        f = 3,

        g = 4,

        h = 5,

        i = 6;

    

    a!==0?b<0?(h=b/a,e=h-1,f=-2*b+2*a*e,i=-2*b+2*a*h,d=2*h*a-2*b-2*a):(h=b/a,e=h+1,f=2*b-2*a*e,i=2*b-2*a*h,d=-2*h*a+2*b):d=h=e=f=i=0;

    

    console.log(a =  + a);

    console.log(b =  + b);

    console.log(d =  + d);

    console.log(e =  + e);

    console.log(f =  + f);

    console.log(g =  + g);

    console.log(h =  + h);

    console.log(i =  + i);

}



test(0);

test(1);

.as-console-wrapper {

  max-height: 100% !important;

}

But wow, I hope this is minified, because if a person wrote that, they must really have a thing against anyone who's supposed to maintain it later... ;-)