How to Use forEach() to Iterate Through a JavaScript Map
JavaScript's Map object has a handy function, forEach(), which operates similarly to arrays' forEach() function.
JavaScript calls the forEach() callback with 3 parameters: the value, the key, and the map itself.
const map = new Map();
map.set('greeting', 'Hello');
map.set('name', 'John');
map.forEach((value, key, map) => {
// Prints "greeting Hello" followed by "name John"
console.log(value, key);
});Map#entries()
JavaScript maps don't have chainable helpers like filter() or map() for arrays.
If you want to use filter() with a map, you should use Map#entries() to first convert the map to an iterator, and then use the the spread operator or the Array.from() function to convert the iterator to an array.
const map = new Map();
map.set('greeting', 'Hello');
map.set('name', 'John');
[...map.entries()]; // [['greeting', 'Hello'], ['name', 'John']]
Array.from(map.entries()); // [['greeting', 'Hello'], ['name', 'John']]
// Equivalent since `entries()` is the default iterator
[...map]; // [['greeting', 'Hello'], ['name', 'John']]
// First convert map into an array of entries, then you can use `filter()`
[...map.entries()].filter(([key, value]) => value.length > 4); // [['greeting', 'Hello']]Map#keys() and Map#values()
If you only need the keys or the values of the Map, you can use Map#keys() or Map#values().
Map#keys() returns an iterator over the map's keys, and Map#values() returns an iterator over the map's values.
Make sure you convert the iterator to an array using the spread operator or Array.from() if you want to use filter() or map()!
const map = new Map();
map.set('greeting', 'Hello');
map.set('name', 'John');
Array.from(map.keys()); // ['greeting', 'name']
Array.from(map.values()); // ['Hello', 'John']