const asyncMap = (array, fn, callback) => {
  let sqArray = [];
  array.map((number, index) => {
    sqArray.push(
      new Promise((resolve, reject) => {
        fn(number, (error, sqNum) => {
          if (!error) {
            resolve(sqNum);
          }
        });
      })
    );
  });
  Promise.all(sqArray).then(response => {
    callback(null, response);
  });
};

asyncMap(
  [1, 2, 3, 4],
  (number, callback) => {
    setTimeout(() => {
      if (typeof number !== "number") {
        callback(new Error(`Not a number: ${number}`));
        return;
      }

      callback(null, number * number);
    }, Math.floor(Math.random() * 100));
  },
  (error, result) => {
    if (error) {
      throw error;
    }

    // Should print [ 1, 4, 9, 16 ]
    console.log(result);
  }
);