What is hashtable in javascript and in what scenarios one must decide to use hashtable as a data structure ?

I would add that Map and Set come standard with modern JS, which enables you to create hashtables out of object references, not just strings. This allows you to pass any object, primitive type, null or undefined as the key with any other value. As a bonus, they both expose Iterator interfaces.

const a = {}; // this is one object with its own ref
const b = {}; // this is another object with a different ref

const myMap = new Map();
myMap.set(a, 'object a');
myMap.set(b, 'object b');

myMap.has({}); // returns `false`
myMap.has(a);  // returns `true`
myMap.get(b);  // returns 'object b'

const mySet = new Set();
mySet.add(a);

mySet.has(a); // returns `true`
mySet.has(b); // returns `false`
mySet.has({}); // returns `false`

// this will print out 'object a' and 'object b'
for (let [key, value] of myMap) {
  console.log(value);
}

You're probably talking about an object const hashtable = {}.

In JS, everything is an object (like in Ruby), and here you can store any key/value pair.

Objects are very useful in any occasion unless you just need a single variable or a list (array). Of course I'm not extending to classes and O.

You access object's data using the key you need const age = person.age //or person['age'] and set it's value the same way person.name = "john" //person["name"] = "john".