Map.prototype.forEach()

map.forEach(callbackFn, thisArg)

Returns: undefined · Updated March 13, 2026 · Map and Set

map foreach iteration

The forEach() method executes a provided function once for each key-value pair in the Map. The callback receives the value, key, and the Map itself as arguments.

Syntax

map.forEach(callbackFn, thisArg)

Parameters

Parameter	Type	Default	Description
`callbackFn`	`function`	—	Function called for each key-value pair
`value`	`any`	—	The current value
`key`	`any`	—	The current key
`map`	`Map`	—	The Map being iterated
`thisArg`	`any`	`undefined`	Value to use as `this`

Examples

Basic usage

const prices = new Map([
  ["apple", 1.5],
  ["banana", 0.75],
  ["orange", 2.0]
]);

prices.forEach((value, key) => {
  console.log(`${key}: $${value}`);
});
// apple: $1.5
// banana: $0.75
// orange: $2.0

Using thisArg

const multiplier = {
  factor: 2
};

const nums = new Map([["a", 1], ["b", 2], ["c", 3]]);

nums.forEach(function(value, key) {
  console.log(`${key}: ${value} x ${this.factor} = ${value * this.factor}`);
}, multiplier);
// a: 1 x 2 = 2
// b: 2 x 2 = 4
// c: 3 x 2 = 6

Calculating totals

const orders = new Map([
  ["order1", 100],
  ["order2", 250],
  ["order3", 75]
]);

let total = 0;
orders.forEach((value) => {
  total += value;
});
console.log(`Total: $${total}`); // Total: $425

Transforming to another structure

const users = new Map([
  [1, { name: "Alice" }],
  [2, { name: "Bob" }],
  [3, { name: "Carol" }]
]);

const names = [Global_Objects::eval];
users.forEach((value, key) => {
  names.push(`${key}: ${value.name}`);
});
console.log(names); // [1: Alice, 2: Bob, 3: Carol]

Filtering during iteration

const scores = new Map([
  ["player1", 95],
  ["player2", 82],
  ["player3", 78],
  ["player4", 91]
]);

const highScorers = [Global_Objects::eval];
scores.forEach((value, key) => {
  if (value >= 90) {
    highScorers.push(key);
  }
});
console.log(highScorers); // [player1, player4]

Modifying values in place

const inventory = new Map([
  ["apples", 10],
  ["bananas", 5],
  ["oranges", 8]
]);

inventory.forEach((value, key, map) => {
  map.set(key, value * 2);
});

console.log([...inventory]);
// [[apples, 20], [bananas, 10], [oranges, 16]]

With async operations

const apiCalls = new Map([
  ["user", "/api/user/1"],
  ["posts", "/api/posts"],
  ["comments", "/api/comments"]
]);

async function fetchAll() {
  const results = new Map();
  
  for (const [key, url] of apiCalls) {
    // Simulated fetch
    results.set(key, { url, status: "loaded" });
  }
  
  return results;
}

fetchAll().then(data => console.log([...data.entries()]));

Behavior Notes

Iteration order follows insertion order
No way to break out of forEach — use for...of instead if you need early exit
forEach() always returns undefined