Javascript order of referencing

Question

What order does the following statement run? Does the runtime execute it from right to left?

  length = i = test = output = null;

Answer 1

That single line assignment is equivalent to this

output = null;
test = output;
i = test;
length = i;

Answer 2

Yes, it executes from right to left. The value of everything is now null.

Answer 3

Does the runtime execute it from right to left?

Yes.

foo = {
    set a() {
        console.log('a');
    },
    set b() {
        console.log('b');
    },
    set c() {
        console.log('c');
    },
    set d() {
        console.log('d');
    }
};
foo.a = foo.b = foo.c = foo.d = 'bar';

produces:

d
c
b
a

in the console. This order is necessary because each assignment relies on the return value of the previous assignment:

foo.a = foo.b = foo.c = foo.d = 'bar';

is equivalent to:

foo.a = (foo.b = (foo.c = (foo.d = 'bar')));

However it is not equivalent to:

foo.d = 'bar';
foo.c = foo.d;
foo.b = foo.c;
foo.a = foo.b;

The return value of a = b is b. This is especially important to remember should you choose to implement accessors and mutators.

What this means is that my previous example is equivalent to:

foo.d = 'bar';
foo.c = 'bar';
foo.b = 'bar';
foo.a = 'bar';

Answer 4

Yes, since an assignment expression is not a lefthand-side expression itself, the nesting is

(length = (i = (test = (output = null))));

It is executed from outside-in, solving references left-to-right (first getting the length variable, then evaluating the right side, which is getting the i variable and evaluating …). That means output is the first variable that is getting assigned to, then test, then i, and last length: "right-to-left" if you want. Since the assignment operator always yields its right operand, everything will be assigned the same (innermost, rightmost) value: null.