JavaScript Maps — Interview Questions & Answers

Ques: What is a Map in JavaScript?

Ans: A Map is a collection of key-value pairs, similar to objects — but with improved capabilities.

Unlike objects, keys in Maps can be of any type (not just strings or symbols).

const map = new Map();
map.set('name', 'John');
map.set(100, 'score');
map.set(true, 'isActive');

Ques: Why use Map instead of Object?

Ques: How do you create a Map?

Ans: Using the new Map() constructor:

const user = new Map([
  ['name', 'Alice'],
  ['age', 25]
]);

Ques: How do you add or update values in a Map?

Ans: Use the .set() method:

const map = new Map();
map.set('color', 'blue');
map.set('color', 'red'); // updates existing key

Ques: How do you retrieve a value from a Map?

Ans: Use .get(key):

console.log(map.get('color')); // red

Ques: How do you check if a key exists in a Map?

Ans: Use .has(key):

console.log(map.has('color')); // true

Ques: How do you remove a key-value pair?

Ans: Use .delete(key) or .clear() to empty all entries:

map.delete('color');
map.clear();

Ques: How do you find the number of entries in a Map?

Ans: Use the .size property:

console.log(map.size);

Ques: How do you iterate through a Map?

for (let [key, value] of map) {
  console.log(`${key}: ${value}`);
}

Or use .forEach():

map.forEach((value, key) => console.log(key, value));

Ques: How do you convert a Map to an Array?

const arr = Array.from(map);
console.log(arr); // [[key, value], [key, value]]

Ques: How do you convert an Object to a Map?

const obj = { name: 'John', age: 30 };
const map = new Map(Object.entries(obj));

Ques: How do you convert a Map back to an Object?

const obj = Object.fromEntries(map);

Ques: What are the main Map methods?

Ques: Example: Iterating keys and values separately

const fruits = new Map([
  ['apple', 10],
  ['banana', 20],
]);

for (let key of fruits.keys()) console.log(key);
for (let value of fruits.values()) console.log(value);

Ques: Are Maps ordered?

Ans: Yes, Maps maintain insertion order, unlike regular objects.

Ques: Can Map keys be objects?

Ans: Yes, any object or even a function can be a key.

const objKey = { id: 1 };
const map = new Map();
map.set(objKey, 'User Data');
console.log(map.get(objKey)); // User Data

Ques: What happens if you use the same key twice in a Map?

Ans: It overwrites the previous value — keys are unique.

Ques: Difference between Map and WeakMap

Ques: What is a WeakMap?

Ans: A WeakMap stores key-value pairs where keys must be objects and are weakly referenced (garbage-collected if no other references exist).

let obj = { id: 1 };
const wm = new WeakMap();
wm.set(obj, "Secret Info");
obj = null; // key-value pair can be garbage collected

Ques: Can you loop through a WeakMap?

Ans: No, WeakMap is not iterable because its keys are weakly held and can disappear anytime.

Ques: Use cases of Map

• Caching data results (like API responses)
• Storing metadata for DOM elements
• Counting frequency of items
• Maintaining insertion order of dynamic data

Ques: Use cases of WeakMap

• Private data storage for class instances
• Avoiding memory leaks with temporary object references

Ques: Example: Counting word frequency using Map

const text = "apple banana apple orange banana apple";
const words = text.split(" ");
const count = new Map();

words.forEach(word => {
  count.set(word, (count.get(word) || 0) + 1);
});

console.log(count);
// Map { 'apple' => 3, 'banana' => 2, 'orange' => 1 }