Javascript Data Structure Dictionary

function Dictionary() { 
   this.add = add; // www. j  ava2  s  .  com
   this.datastore = new Array(); 
   this.find = find; 
   this.remove = remove; 
   this.showAll = showAll; 
   this.count = count; 
   this.clear = clear; 
} 

function add(key, value) { 
   this.datastore[key] = value; 
} 

function find(key) { 
   return this.datastore[key]; 
} 

function remove(key) { 
   delete this.datastore[key]; 
} 

function showAll() { 
   for  (let key in Object.keys(this.datastore)) { 
      console.log(key + " -> " + this.datastore[key]); 
   } 
} 

function count() { 
   let n = 0; 
   for (let key in Object.keys(this.datastore)) { 
      ++n; 
   } 
   return n; 
} 

function clear() { 
   for (let key in Object.keys(this.datastore)) { 
      delete this.datastore[key]; 
   } 
} 


let pbook = new Dictionary(); 
pbook.add("Raymond","123"); 
pbook.add("David", "345"); 
pbook.add("Cynthia", "456"); 
console.log("Number of entries: " + pbook.count()); 
console.log("David's extension: " + pbook.find("David")); 
pbook.showAll(); 
pbook.clear(); 
console.log("Number of entries: " + pbook.count());