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

Why? How can we fix the code to make keys.push work?

That’s because map.keys() returns an iterable, but not an array.

We can convert it into an array using Array.from:

let map = new Map();
map.set("name", "John");
let keys = Array.from(map.keys());
keys.push("more");
alert(keys); // name, more