OK, let's go a bit more comprehensive and start with what's Map for those who don't know this feature in JavaScript... MDN says:
The Map object holds key-value pairs and remembers the original insertion order of the keys.
Any value (both objects and primitive values) may be used as either a key or a value.
As you mentioned, you can easily create an instance of Map using new keyword... In your case:
let myMap = new Map().set('a', 1).set('b', 2);
So let's see...
The way you mentioned is an OK way to do it, but yes, there are more concise ways to do that...
Map has many methods which you can use, like set() which you already used to assign the key values...
One of them is keys() which returns all the keys...
In your case, it will return:
MapIterator {"a", "b"}
and you easily convert them to an Array using ES6 ways, like spread operator...
const b = [...myMap.keys()];