https://javascript.info/map-set

jst_M&S_iterableKeys.js

// Iterable keys
// importance: 5
// We’d like to get an array of map.keys() in a variable and then apply array-specific methods to it, e.g. .push.

// But that doesn’t work:

let map = new Map();

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

let keys = Array.from(map.keys());

// Error: keys.push is not a function
keys.push("more");

console.log(keys);