Mapping.js

class Mapping {
    // Subclasses define at least @@iterator().
    @@iterator() {
        throw TypeError("abstract operation")
    }

    // Mutable subclasses also define set() and delete().
    set(key, value) {
        throw TypeError("mapping is not mutable");
    }
    delete(key) {
        throw TypeError("mapping is not mutable");
    }

    // The other methods are defined in terms of the basic operations above.
    // But typically a subclass will also define at least get(), has(), and size, for performance.
    get size() {
        let n = 0;
        for (let _ of this)
            n++;
        return n;
    }
    get(key) {
        for (let [k, v] of this)
            if (sameValue(k, key))
                return v;
    }
    has(key) {
        for (let [k, v] of this)
            if (sameValue(k, key))
                return true;
        return false;
    }
    clear() {
        for (let k of [...this.keys()])
            this.delete(k);
    }
    *entries() {
        for (let pair of this)
            yield pair;
    }
    *keys() {
        for (let [k, _] of this)
            yield k;
    }
    *values() {
        for (let [_, v] of this)
            yield v;
    }
    forEach(callbackFn, thisArg) {
        // This would be specified in terms of [[Call]] rather than the .call method.
        for (let [k, v] of this)
            callbackFn.call(thisArg, v, k, this);
    }
}