Map iteration

Learn to iterate a Map with for...of, entries and forEach.

Looping over a Map

When you loop over a Map with for...of, each step gives you a [key, value] array. You can destructure it: for (const [key, value] of map).

Map iterates keys in insertion order; this order is always consistent. Loop controls like break and continue work with Map as usual.

entries, keys, values, forEach

Map has three iterator methods: entries() returns [key, value] pairs, keys() returns just keys, and values() returns just values. forEach((value, key) => {...}) runs a callback for each entry.

Note: in forEach the parameter order is value first, then key. This matches array forEach but differs from Map's [key, value] order.

Exercise Iterate a Map with for...of

Create a Map called prices. Add 5 under key "apple" and 7 under key "banana". Loop over the Map with for...of using [item, price] destructuring and log each pair with console.log(item, price).

Show example solution

const prices = new Map();
prices.set("apple", 5);
prices.set("banana", 7);
for (const [item, price] of prices) {
  console.log(item, price);
}

Test yourself

In `for (const x of map)`, what is `x`?
- Only the key
- Only the value
- The Map itself
- A [key, value] array
Show answer

A [key, value] array — Correct. for...of yields a [key, value] pair at each step by default.

for...of destructuring

const prices = new Map();
prices.set("apple", 5);
for (const [item, price] ____ prices) {
  console.log(item, price);
}

Show answer

const prices = new Map();
prices.set("apple", 5);
for (const [item, price] of prices) {
  console.log(item, price);
}

What is the parameter order in Map forEach?
- (key, value)
- (value, key)
- only value
- only key
Show answer

(value, key) — Correct. Map forEach passes value first, then key.
Match the Map iteration methods

entries()

[key, value] pairs
keys()

Only keys
values()

Only values
forEach

Callback iteration

Next lesson: WeakMap and WeakSet

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