How to iterate a Map in Javascript - Map Part 2

js.Srijan

Table of Contents

Introduction

If you missed reading the first article in which we discussed how to do basic operations over a Map, you can read it here.

In this article, we will learn how to iterate over a Map using different methods available. So, let’s dive in and learn how to loop over a Map.

Iterating a Map

For iterating over a Map, we can use the following javascript constructs:

  • for..of
  • forEach()

Let's create a Map first to loop over from the knowledge gained in our previous article.

let map = new Map() map.set("one", "first element"); map.set("two", "second element"); map.set(3, "third element");

Iterating map with for..of

We can use for..of to iterate a Map in Javascript.

for (let [key, value] of map) { console.log(key + " = " + value); } //output // one = first element // two = second element // 3 = third element

Map also provides these three methods, which comes in handy while iterating over a Map.

  • map.keys() - Returns an iterable for keys
  • map.values() - Returns an iterable for values
  • map.entries() - Returns an iterable of [key,value]
for (let key of map.keys()) { console.log(key); } // output // one // two // 3 for (let value of map.values()){ console.log(value); } // output // first element // second element // third element for (let [key, value] of map.entries()) { console.log(key + " = " + value) } //output // one = first element // two = second element // 3 = third element

Iterating Map with forEach()

We can also iterate through a Map using forEach().

map.forEach(function(value, key) { console.log(key + " = " + value); }) //output // one = first element // two = second element // 3 = third element

Map preserves the order in which values are inserted. So, while iterating over a Map, elements will be in the same order in which they are inserted.

Summary

  • To iterate over a Map, we can use for..of and forEach() loop constructs.
  • Map provides three methods that return iterable: map.keys(), map.values() and map.entries().
  • Iteration over Maps is always in insertion order.

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