Map vs object for dictionary

map-dict.js

// A Dict class that works in ES6 using Map. Basically it's identical to Map, but
// only expected to be used on string keys.

function Dict() {
    this.elts = new Map();
}

// (string) -> any
Dict.prototype.get = function get(key) {
    return this.elts.get(key);
};

// (string, any) -> void
Dict.prototype.set = function set(key, val) {
    return this.elts.set(key, val);
};

// (string) -> boolean
Dict.prototype.has = function has(key) {
    return this.elts.has(key);
};

// (string) -> void
Dict.prototype.remove = function remove(key) {
    this.elts.delete(key);
};

object-dict.js

// A Dict class that works in ES5, or in engines that support __proto__ so that
// a polyfilled Object.create can create prototype-less objects. This hasn't been
// tested or optimized at all. I'll accept any optimizations so long as it's
// never sensitive to prototype pollution (e.g. in Object.prototype).

function Dict() {
    this.dunderProto = Object.create(null);
    this.elts = Object.create(null);
}

// (string) -> any
Dict.prototype.get = function get(key) {
    return (key === '__proto__') ? this.dunderProto.value : this.elts[key];
};

// (string, any) -> void
Dict.prototype.set = function set(key, val) {
    if (key === '__proto__') {
        this.dunderProto.value = val;
    } else {
        this.elts[key] = val;
    }
};

// (string) -> boolean
Dict.prototype.has = function has(key) {
    return (key === '__proto__') ? 'value' in this.dunderProto : key in this.elts;
};

// (string) -> void
Dict.prototype.remove = function remove(key) {
    if (key === '__proto__') {
        delete this.dunderProto.value;
    } else {
        delete this.elts[key];
    }
};