JSFiddle - React, Tailwind, and code Playground

JavaScript

function HashMap() {}

HashMap.prototype.put = function(key, value) {
    this[key] = value;
};

HashMap.prototype.get = function(key) {
    if(typeof this[key] == 'undefined') {
        throw new ReferenceError("key is undefined");
    }
    return this[key];
};

HashMap.prototype.size = function() {
    var count = 0;

    for(var prop in this) {
        if(this.hasOwnProperty(prop)) {
            count++;
        }
    }

    return count;
};

// test
var map = new HashMap();

map.put("a", 1);
map.put("b", 2);

alert(map.size()); // 2