How does a.x = a = {n: b} work in JavaScript?

Question

This is related to Javascript a=b=c statements.

I do understand that

foo = foo.x = {n: b}; // console.log(foo) => {n: b}

but

Answer 1

It equals

let tmp = foo;
foo = {n: b};
tmp.x = foo;

You could see, that old foo (stored in z in this example) was modified:

> z=foo={};
{}
> foo.x = foo = {n: b};
{ n: 10 }
> foo
{ n: 10 }
> z
{ x: { n: 10 } }