How does [b][b = a,0] swap between a and b?

Question

As gdoron pointed out,

var a = \"a\";
var b = \"b\";

a = [b][b = a,0];

Will swap a and b, and although it looks

Answer 1

It might help (or hinder) to think of it terms of the semantically equivalent lambda construction (here, parameter c takes the place of element 0):

a = (function(c) { b = a; return c; })(b);