A simple map implementation for JavaScript (not intended to be an ES6 polyfill)

simplemap.js

function SimpleMap() {  
    this._data = {};
}

SimpleMap.prototype = {
    
    get: function(key) {
        return this.has(key) ? this._data[key] : null;
    },

    has: function(key) {
        return Object.prototype.hasOwnProperty.call(this._data, key);
    },

    set: function(key, value) {
        this._data[key] = value;
    }
};

@fitzgen that can break regardless so a more robust version could be:

function SimpleMap() {  
    this._data = {};
}

SimpleMap.prototype = {

    get: function(key) {
        return this.has(key) ? this._data['@' + key] : void 0;
    },

    has: function(key) {
        return this.hasOwnProperty.call(this._data, '@' + key);
    },

    set: function(key, value) {
        this._data['@' + key] = value;
    }
};

Although this is nothing better than just Object.create(null) but yeah, latter one might need a little polyfill

My goal in creating this was for a project that had to work in IE < 9, so I couldn't use Object.create().

I like @WebReflection's alternative.