How to detect combined keystroke (Alt + single-digit or multi-digit number) and get the number?

I'm trying to detect a combined keystroke consisting of Alt (held down) and a single-digit or multi-digit number and then retrieve the entered number for further processing. For example:

Input: Alt + 4 2

Desired output: 42

I guess I need to combine keydown and keyup together and store the entered digits in an array and then generate a number from them. How to achieve this?

You indeed shoulod take advantage of events keydown and keyup. We're using a global object to remember all the keys that are down, because they will fire different events.

var obj_down = {}
var digits = []

window.addEventListener("keydown", function(ev) {
  obj_down[ev.key] = true;
  if (obj_down["Alt"] === true) {
    if (ev.key >= '0' && ev.key <= '9') {
      digits.push(+ev.key)
    }
  }
})

window.addEventListener("keyup", function(ev) {
  delete obj_down[ev.key];
  if (ev.key === 'Alt') {
    document.querySelector(".output").innerText = digits.join("")
    digits = []
  }
})

<p>hold alt and type a number</p>
<input type="text">
<br> output:
<div class="output">