1002010 - Subclassing Map fails

Status: RESOLVED DUPLICATE of bug 838540

(Core :: JavaScript Engine, defect)

Description

[aleth [:aleth]]

If one tries to subclass a Map using Object.create, one runs into the following problem:

    function newMap(args) {
      Map.call(this, args);
    }
    newMap.prototype = Object.create(Map.prototype);
    newMap.prototype.constructor = newMap;
     
    let testmap = new newMap([["a",1],["b",2]]);
    print(testmap.get("b"));
    /*
    Exception: get method called on incompatible Object
    */ 

This despite newMaps being instances of Maps.

It's possible this is in fact not a bug, but if so, I'd like to understand why (and it probably should be explained on MDN too).

Comment 1

[Boris Zbarsky [:bzbarsky]]

SpiderMonkey doesn't have ES6 subclassing support (and in particular support for @@create) yet.  So "this" in newMap ends up as a plain Object without the internal slots a Map needs.