1394626 - JS engine confuses undefined with empty in completion records

Status: RESOLVED DUPLICATE of bug 1317378

(Core :: JavaScript Engine, defect)

Description

[Oriol Brufau [:Oriol]]

Run this code: eval('1; if(0){}')
Result: 1
Expected: undefined

If I understand correctly,

1. `1;` returns a normal completion record with value `1`, let's call it `sl`.
2. `if(0){}` returns `NormalCompletion(undefined)`, let's call it `s`, according to http://www.ecma-international.org/ecma-262/8.0/#sec-if-statement-runtime-semantics-evaluation.
3. Therefore `1; if(0){}` returns `Completion(UpdateEmpty(s, sl))` according to http://www.ecma-international.org/ecma-262/8.0/#sec-block-runtime-semantics-evaluation.
4. `UpdateEmpty(s, sl)` returns `Completion(s)` because `s.[[Value]]` is not `empty` (it's `undefined`), according to http://www.ecma-international.org/ecma-262/8.0/#sec-updateempty.

It seems the JS engine thinks that `undefined` is `empty` and thus returns `Completion{[[Type]]: s.[[Type]], [[Value]]: sl, [[Target]]: s.[[Target]] }`

Edge behaves like Firefox, Chrome does it correctly.