JavaScript code trick: What's the value of foo.x
foo.x = foo = {n: 2};
determines that foo.x
refers to a property x
of the {n: 1}
object, assigns {n: 2}
to foo
, and assigns the new value of foo
– {n: 2}
– to the property x
of the {n: 1}
object.
The important thing is that the foo
that foo.x
refers to is determined before foo
changes.
See section 11.13.1 of the ES5 spec:
Let lref be the result of evaluating LeftHandSideExpression.
Let rref be the result of evaluating AssignmentExpression.
The assignment operator associates right to left, so you get:
foo.x = (foo = {n: 2})
The left hand side is evaluated before the right hand side.
foo.x = foo = {n: 2};
Here foo refers to {n:1} object before assignment i.e. before the statement is executed.
The statement can be re-written as foo.x = (foo = {n:2});
In object terms the above statement can be re-written as {n:1}.x = ( {n:1} = {n:2} );
Since assignment happens from right to left only. So here we just have to keep a check that foo is referring to which object before execution starts.
On solving the R.H.S: foo = {n:2}; Now foo is referring to {n:2};
Coming back on the problem we are left with:
foo.x = foo;
Now foo.x on L.H.S is still {n:1}.x whereas foo on R.H.S is {n:2}.
So after this statement gets executed {n:1} will become { n:1, x:{n:2} } with bar still referring to it. Where as foo will now be referring to {n:2}.
So on execution foo.x gives undefined as there is only 1 value in foo which is {n:2}.
But if you will try executing bar.x it will give {n:2}. Or if you will just execute bar the result will be
Object {n: 1, x: Object}
As I understand expression :
foo.x = foo = {n: 2};
just exactly the same as:
foo.x = {n: 2} ;
foo = {n: 2};
And after this it's became obvious that:
bar=={n: 1, x: {n:2}};
foo=={n:2};
foo.x==undefined