Multiple left-hand assignment with JavaScript, really right associative? [duplicate]

Answer 1

tl;dr — JS works out where to put the value before working out what that value is, and a side effect of working out what that value is changes the value of a.

---

See the spec for simple assignment.

Step 1 is "Let lref be the result of evaluating LeftHandSideExpression."

Step 2 is "Let rref be the result of evaluating AssignmentExpression."

So the first thing that happens is that a property x is created on the object stored in a (where n is 1).

Then the right hand side is evaluated (which ends up overwriting a with a new object where n is 2).

Then the result of that expression (that object where n is 2) is assigned to x on the original object (where n is 1).

You can see this in effect with:

"use strict";
var a = {n: 1};
var b = a;

a.x = a = {n: 2};

console.log(a);
console.log(b);