According to the spec, the left hand side of an assignment expression is evaluated first, even though the operator has right-to-left precedence. Thus the expression foo.x = foo = {n: 2} which is the same as foo.x = (foo = {n: 2}) is evaluated like this:
- Evaluate the left-hand expression
foo.x to get a reference, which is where the value of the right-hand expression will be assigned to.
- Evaluate the right-hand expression, to to get the value that will be assigned. The right-hand side is another assignment expression, so it gets evaluated the same way:
- Evaluate
foo to determine where to assign to.
- Evaluate the expression
{n:2}, which creates an object, to determine the value to assign.
- Assign
{n:2} to foo, and return {n:2}.
- Assign the value that the expression on the right-side evaluated to (
{n:2}), to the reference that foo.x resolved to in step 1 (before foo was assigned a new value). Which is also the same as bar.x, because of the assignment bar = foo on the line before.
When this is done, the original object, that bar is still a reference to, will have an x property that references the second object created. foo is also a reference to that second object, so foo === bar.x.
