Why does this JavaScript work?

Question

I was looking at the output of some stuff from UglifyJS and happened across some code like the following:

var a = 0;
var b = function () {
    return function () {
        a++;
    }(), 'Hello, World'
}();

After running that code a is 1 and b is the string Hello, World!.

At first glance it appears that b will be undefined since it looks like the result of a function with no return value is being returned, but that is followed by a comma and a string literal.

Why does this work?
And why doesn't UglifyJS just execute the anonymous function and then return Hello, World! as the next statement?

Answer 1

It works due to the comma operator. From the MDN specs:

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

Both functions are IFFYs, they are inmediately executed.

Answer 2

The result of an expression using the comma operator is the right hand side of the comma operator.

You have:

return a_function_call(), a_string

… so you get a_string assigned.