How to reliably check an object is an EcmaScript 6 Map/Set?

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The situation is similar to pre-ES5 methods to detect arrays properly and reliably. See this great article for the possible pitfalls of implementing isArray.

We can use

  • obj.constructor == Map/Set, but that doesn’t work on subclass instances (and can easily be deceived)
  • obj instanceof Map/Set, but that still doesn’t work across realms (and can be deceived by prototype mangling)
  • obj[Symbol.toStringTag] == "Map"/"Set", but that can trivially be deceived again.

To be really sure, we’d need to test whether an object has a [[MapData]]/[[SetData]] internal slot. Which is not so easily accessible – it’s internal. We can use a hack, though:

function isMap(o) {
    try {
        Map.prototype.has.call(o); // throws if o is not an object or has no [[MapData]]
        return true;
    } catch(e) {
        return false;
    }
}
function isSet(o) {
    try {
        Set.prototype.has.call(o); // throws if o is not an object or has no [[SetData]]
        return true;
    } catch(e) {
        return false;
    }
}

For common use, I’d recommend instanceof – it’s simple, understandable, performant, and works for most reasonable cases. Or you go for duck typing right away and only check whether the object has has/get/set/delete/add/delete methods.

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