You can use the spread operator to convert Map.keys() iterator in an Array.

let myMap = new Map().set('a', 1).set('b', 2).set(983, true)
let keys = [...myMap.keys()]
console.log(keys)

It's not a map, it's an object. (You might consider using a Map, though.)

To get its properties as key/value pairs, you can use Object.entries, which you can then apply map to:

map = Object.entries(map).map(([key, value]) => key + "_" + value);

Object.entries is relatively new, but easily polyfilled for older environments.

Live Example:

var map = {"a": 1, "b": 2, "c": 3};
map = Object.entries(map).map(([key, value]) => key + "_" + value);
console.log(map);

Or, using a Map, you can use its built-in entries method, which returns an iterable, passing it into Array.from and using Array.from's mapping callback:

var map = new Map([
  ["a", 1],
  ["b", 2],
  ["c", 3]
]);
map = Array.from(map.entries(), ([key, value]) => key + "_" + value);
console.log(map);

(Or expand the iterable into an array — [...map.entries()] — and use map on it, but the above avoids a temporary throw-away array.)

In both cases, I'm using destructuring in the parameter list of the arrow function, which receives an array in [key, value] format.

It's Array.prototype.map actually, it's defined for arrays, so use Array.from to convert the keys to an array and then use map:

const map = new Map();

map.set(0, 'Zero');
map.set(1, 'One');
map.set(2, 'Two');

console.log(...Array.from(map.keys()).map(key => {
  return key ** 2; // square the keys
}));

JSBench with the methods as per the following. These are the results from fastest to slowest:

1. Map.prototype.forEach()
2. for of loop through Map.prototype.keys()
3. for of loop through Map.prototype[Symbol.iterator]()
4. Create array from Map.prototype.keys() and then Array.prototype.forEach()

You could convert it to an array of entries (using [...people.entries()]) and search for it within that array.

let people = new Map();
people.set('1', 'jhon');
people.set('2', 'jasmein');
people.set('3', 'abdo');
    
let jhonKeys = [...people.entries()]
        .filter(({ 1: v }) => v === 'jhon')
        .map(([k]) => k);

console.log(jhonKeys); // if empty, no key found otherwise all found keys.