https://eloquentjavascript.net/11_async.html#i_Ug+Dv9Mmsw

Eloquent JavaScript 1102 Building Promise.all.js

function Promise_all(promises) {
  return new Promise((resolve, reject) => {
    const array = [];
    
    const promiseHelper = (promises) => {
      if (promises.length === 0) {
      	resolve(array); // return array;
      }
      
      promises[0]
        .then((value) => {
          array.push(value);
          promiseHelper(promises.slice(1));
      	})
      	.catch(reject);
    };
    
    promiseHelper(promises);
  });
}

// Test code.
Promise_all([]).then(array => {
  console.log("This should be []:", array);
});
function soon(val) {
  return new Promise(resolve => {
    setTimeout(() => resolve(val), Math.random() * 500);
  });
}
Promise_all([soon(1), soon(2), soon(3)]).then(array => {
  console.log("This should be [1, 2, 3]:", array);
});
Promise_all([soon(1), Promise.reject("X"), soon(3)])
  .then(array => {
    console.log("We should not get here");
  })
  .catch(error => {
    if (error != "X") {
      console.log("Unexpected failure:", error);
    }
  });