1107418 - Destructuring assignment of function arguments object is broken

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Oleg Torbasow]

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0
Build ID: 20141113143407

Steps to reproduce:

(function test(a, b){let [x, y]=arguments;})(1,2);


Actual results:

undefined


Expected results:

TypeError: arguments['@@iterator'] is not a function

Comment 1

[Oleg Torbasow]

This works fine in Firefox 15, 21, 24, 25, 30, 32 and 33, although arguments was not iterable at Fx24 yet (bug 1067049). Try (function test(a, b){for(x of arguments);})(1,2). This works in Fx15 and doesn’t work in Fx24 and later.

Comment 2

[[:Aleksej]]

2014-12-07-03-02-05-mozilla-central-firefox-37.0a1.ru.linux-x86_64

(function test(a, b){let [x, y]=arguments;})(1,2);
Exception: arguments[Symbol.iterator] is not a function

(function test(a, b){for(x of arguments);})(1,2)
Exception: arguments[Symbol.iterator] is not a function

Comment 3

[:Gijs (he/him)]

Jan, was this a deliberate change? I don't know enough about the spec here to know if this is supposed to work...

Comment 4

[Jan de Mooij [:jandem]]

Problem is that `arguments` object don't have a Symbol.iterator property. In the spec I don't see @@iterator defined for arguments objects but maybe I'm missing something...

Comment 5

[Boris Zbarsky [:bzbarsky]]

Per spec, "let [x, y] = foo" iterates foo twice and assigns the results to x and y.

Also per spec afaict arguments objects should be iterable, but we haven't fixed bug 1067049 yet.  Presumably we should just fix that.

Comment 6

[Boris Zbarsky [:bzbarsky]]

> In the spec I don't see @@iterator defined for arguments objects 

It's in step 22 of http://people.mozilla.org/~jorendorff/es6-draft.html#sec-createmappedargumentsobject and step 7 of http://people.mozilla.org/~jorendorff/es6-draft.html#sec-createunmappedargumentsobject

Comment 7

[Jan de Mooij [:jandem]]

Ah thanks Boris. Not sure how I missed that when I searched for @@iterator...

Comment 8

[Jan de Mooij [:jandem]]

Yup, this works now that bug 1067049 is fixed.