HashMap in JavaScript

In JS, a HashMap is implemented using:

Map (preferred modern way)
plain object {} (older way)

Using Map (Recommended)

Create Map

const map = new Map();

Basic Operations

set() insert/update

map.set("name", "John");
map.set("age", 25);

Time: O(1)

get() retrieve

map.get("name"); // "John"

Time: O(1)

has() check existence

map.has("age"); // true

Time: O(1)

delete()

map.delete("age");

Time: O(1)

size

map.size;

Example Walkthrough

const freq = new Map();

const arr = [1, 2, 2, 3];

for (let num of arr) {
  freq.set(num, (freq.get(num) || 0) + 1);
}

console.log(freq);
// Map {1 => 1, 2 => 2, 3 => 1}

Very common interview pattern