object map function

keyMap.js

let keyMap = obj => (callback, formatter) => {
    let keys = Object.keys(obj),
        result = [];

    for (let i = 0; i < keys.length; i++) {
        let key = keys[i];
        result[typeof formatter === 'function' ? formatter(key) : i] = callback(obj[key], key);
    }

    return result;
};
keyMap({
  a: 1,
  b: 2
})(
  num => num + 1,
  key => "$" + key
);