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?

Feature	Object	Map
Key types	String / Symbol	Any data type
Order	Unordered	Maintains insertion order
Iteration	Complex (Object.keys, etc.)	Easy (forEach, for..of)
Size	Manual count	.size property
Performance	Slower for frequent insert/delete	Optimized for key-value operations

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?

Method	Description
.set(key, value)	Adds/updates a key-value pair
.get(key)	Returns value for the key
.has(key)	Checks key existence
.delete(key)	Removes a key
.clear()	Clears all entries
.size	Returns total number of entries
.keys()	Returns all keys
.values()	Returns all values
.entries()	Returns all key-value pairs

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

Feature	Map	WeakMap
Key Types	Any type	Only objects
Garbage Collection	No	Yes
Iteration	Iterable	Not iterable
Size Property	Yes	No
Use Case	Data storage	Temporary references

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 }