956256 - Access to Map elements using [ ] and for ... in operators

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[KOLANICH]

User Agent: Mozilla/5.0 (Windows NT 6.2; rv:26.0) Gecko/20100101 Firefox/26.0 (Beta/Release)
Build ID: 20131205075310

Steps to reproduce:

let a=new Map();
a["x"]="y";
a["z"]="w";
console.log(a.has("z"));
console.log(a.get("x"));
console.log("--------------");
let a=new Map();
a.set("x","y");
a.set("z","w");
for(let b in a){
    console.log(b);
}


Actual results:

false
undefined
--------------
undefined


Expected results:

true
y
--------------
x
z

Comment 1

[Till Schneidereit [:till]]

This is working as intended. One of the reasons Map and Set don't use the normal object property access mechanisms is that those coerce all keys to strings. Maps and Sets allow you to use objects as keys, with the object identity being used, not their string value. Changing the behavior of object properties would break pretty much all code in existence, so Map and Set need to use a different interface.

As a demonstration of the difference, the following code:

var x = {toString: function() {return "x";}};
var otherX = {toString: function() {return "x";}};
var obj = {};
obj[x] = 'x value';
console.log("object values:");
console.log(obj['x']); // prints 'x value'
console.log(obj[otherX]); // prints 'x value'

var map = new Map();
map.set(x, 'x value');
map.set('x', 'literal x value');
map.set(otherX, 'otherX value');
console.log("map entries:");
for (value of map) // prints three different entries
    console.log(value);

Comment 2

[KOLANICH]

Maybe we should introduce a new operator to access without cast to string, for example [[ ]]

Comment 3

[:Ms2ger (he/him; ⌚ UTC+1/+2)]

You can suggest that on <https://mail.mozilla.org/listinfo/es-discuss>.

Comment 4

[Till Schneidereit [:till]]

Such an operator has been discussed on es-discuss. A quick search yields this thread: https://mail.mozilla.org/pipermail/es-discuss/2011-October/thread.html#17468

In general, this bugtracker is only about our implementation of the language, not about changes to the language's specification.

Comment 5

[KOLANICH]

thanks, will read

Comment 6

[Rick Waldron [:rwaldron]]

(In reply to KOLANICH from comment #2)
> Maybe we should introduce a new operator to access without cast to string,
> for example [[ ]]

This particular syntax is actually valid syntax in since ES3: 

var a = [[1]]; 
a[[0]]; // this will be coerced to a string "0"
// [1]

Because: 

MemberExpression [ Expression ]

Comment 7

[KOLANICH]

< > ?