javascript - Is the arguments object supposed to be an iterable in ES6? - Stack Overflow

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In ES6, I was trying to use the arguments object as an iterable when passed to the Set constructor. It works fine in IE11 and in Chrome 47. It does not work in Firefox 43 (throws a TypeError: arguments is not iterable). I've looked through the ES6 spec and cannot really find a definition of whether the arguments object should be an iterable or not.

Here's an example of what I was trying to do:

function destroyer(arr) {
  var removes = new Set(arguments);
  return arr.filter(function(item) {
    return !removes.has(item);
  });
}

// remove items 2, 3, 5 from the passed in array
var result = destroyer([3, 5, 1, 2, 2], 2, 3, 5);
log(result);

FYI, I know there are various work-arounds for this code such as copying the arguments object into a real array or using rest arguments. This question is about whether the arguments object is supposed to be an iterable or not in ES6 that can be used anywhere iterables are expected.

In ES6, I was trying to use the arguments object as an iterable when passed to the Set constructor. It works fine in IE11 and in Chrome 47. It does not work in Firefox 43 (throws a TypeError: arguments is not iterable). I've looked through the ES6 spec and cannot really find a definition of whether the arguments object should be an iterable or not.

Here's an example of what I was trying to do:

function destroyer(arr) {
  var removes = new Set(arguments);
  return arr.filter(function(item) {
    return !removes.has(item);
  });
}

// remove items 2, 3, 5 from the passed in array
var result = destroyer([3, 5, 1, 2, 2], 2, 3, 5);
log(result);

FYI, I know there are various work-arounds for this code such as copying the arguments object into a real array or using rest arguments. This question is about whether the arguments object is supposed to be an iterable or not in ES6 that can be used anywhere iterables are expected.

• javascript
• ecmascript-6
• iterable

Share Improve this question Follow asked Jan 5, 2016 at 14:57 jfriend00jfriend00 709k104104 gold badges1k1k silver badges1k1k bronze badges 2

• 2 This is a known FF bug: The arguments object should support the ES6 @@iterator protocol – apsillers Commented Jan 5, 2016 at 15:16
• @apsillers - And it looks like the code has already been written to fix it (last month) and it is in the process of being reviewed and tested. – jfriend00 Commented Jan 5, 2016 at 15:40

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1 Answer 1

Sorted by: Reset to default 9

I suppose it is a bug in FF's implementation.

As per 9.2.12 FunctionDeclarationInstantiation(func, argumentsList) section,

If argumentsObjectNeeded is true, then

a) If strict is true or if simpleParameterList is false, then

==> i) Let ao be CreateUnmappedArgumentsObject(argumentsList).

b) Else,

==> i) NOTE mapped argument object is only provided for non-strict functions that don’t have a rest parameter, any parameter default value initializers, or any destructured parameters .

==> ii) Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).

Both CreateMappedArgumentsObject and CreateUnmappedArgumentsObject have

Perform DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {[[Value]]:%ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).

It means that @@iterator property should have been actually defined on the arguments object.

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