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javascriptecmascript-harmonyecmascript-6

EcmaScript 6 Map() vs new Map()

To create a map in ES6, should you say Map() or new Map()?

Both seem to work fine in node --harmony.

The draft spec says "The Map constructor is the %Map% intrinsic object and the initial value of the Map property of the global object. When Map is called as a function rather than as a constructor, it initializes its this value with the internal state necessary to support the Map.prototype built-in methods." which also seems to suggest they should both work.

Given that, Map() would seem better because shorter, though that's a subjective judgment; objectively they seem interchangeable?

Solution

  • Although current browsers appear to permit the use of Map() without new (as shown in @RobG's answer), this is in fact incorrect.

    Because of a stability issue, Node.js v0.10 uses an older version of V8 (the JavaScript engine in Node.js) which does not throw if you do Map().

    Node.js v0.10 uses V8 v3.14.5, but the issue with Map() not throwing was only fixed in v3.20.12.

    Chrome, which uses a newer version of V8, throws an error if you don't use new. I'm not sure why Firefox doesn't throw an error though.

    According to the current draft specification, it should be impossible to call Map() without new:

    1. Let map be the this value.
    2. If Type(map) is not Object then, throw a TypeError exception.
    3. If map does not have a [[MapData]] internal slot, then throw a TypeError exception.
    4. If map’s [[MapData]] internal slot is not undefined, then throw a TypeError exception.

    Usually in a constructor the object that is supposed to be created is this. It requires that the object passed is an instance of Map, and that it has not been initialised yet ([[MapData]] must be undefined).

    Apparently this is because it hinders the subclassing of native constructors, which will be possible in ES6.