Why are my keyboard inputs not being registered?

Question

I am making a Snake game using HTML and javascript. The game works by pressing arrow keys to move the snake. It works fine 90% of the time. However, sometimes when I input 2 keys very fast, only one key is registered. For example, if I press the Up and Left button with <10ms delay, the snake will only go Up and not left.

I'm not sure why this is happening so I don't know where to start fixing.

My event listener:

window.onkeydown = function (e){
  var keys = {
    37: 'left',
    38: 'up',
    39: 'right',
    40: 'down'
  }
  var dx = 0;
  var dy = 0;

  switch (keys[e.keyCode]){
    case 'left':
      dx = -1;
      break;
    case 'up':
      dy = 1;
      break;
    case 'down':
      dy = -1;
      break;
    case 'right':
      dx = 1;
      break;
    default:
      return;
  }

Is it possible that my issue will be fixed by changing it to addEventListener?

Another theory I have is that the browser thinks I am inputting 2 keys at the same time, so I'd have to code it to queue the inputs.

Or perhaps Javascript doesn't read inputs when it is doing logic?

Can anyone give me a hint?