1412120 - Array.prototype.join() does not work with "array-like objects" (at least not in all environments)

Status: RESOLVED FIXED

(Developer Documentation Graveyard :: JavaScript, defect)

Description

[darren_adkinson]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36

Steps to reproduce:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join

The first line:

"The join() method joins all elements of an array (or an array-like object) into a string and returns this string."

appears to be incorrect, or at least potentially misleading, as far as certain environments are concerned. It seems necessary to convert arguments to a true array before calling join(), from what I can tell.

Comment 1

[Tooru Fujisawa [:arai]]

https://tc39.github.io/ecma262/#sec-array.prototype.join

per spec, it should work on array-like object.
what's "certain environments" ?

Comment 2

[darren_adkinson]

arguments is an "array-like object" but, afaik, you have to turn it into a true array in order to call 'join' on it. From what I've seen the other array methods don't mention 'or an array-like object' so this seemed to imply there was something about 'join' you could do that you can't with, say, slice.


```
var fn = function (a, b, c) {
  var argList = arguments.join();
  console.log(argList);
}
undefined
fn(1, 2, 3);

Uncaught TypeError: arguments.join is not a function
    at fn (<anonymous>:2:27)
    at <anonymous>:1:1
fn @ VM154:2
(anonymous) @ VM162:1



var fn = function (a, b, c) {
  var argList = [].slice.call(arguments).join();
  console.log(argList);
}
undefined
fn(1, 2, 3);
1,2,3
undefined

```

Comment 3

[Tooru Fujisawa [:arai]]

the implementation of Array.prototype.join works on array-like objects.
then, join is the property of Array.prototype, so you cannot get join property from arguments or other array-like object, unless it has the property.

so, you need to use Function.prototype.call [1] or Function.prototype.apply [2] on Array.prototype.join, like:
  var obj = {
    0: "a",
    1: "b",
    2: "c",
    length: 3
  };
  console.log(Array.prototype.join.call(obj));

or set join property explicitly, like:
  var obj = {
    0: "a",
    1: "b",
    2: "c",
    length: 3,
    join: Array.prototype.join
  };
  console.log(obj.join());

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply

Comment 4

[Tooru Fujisawa [:arai]]

I guess, adding that example may clarify.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join#Joining_an_array-like_object