Javascript fromCharCode keyCode

You could use e.key, it returns value of the pressed key. So if you press left arrow key e.key would be "ArrowLeft" and if you press right arrow key e.key would be "ArrowRight".

You can lookup these codes Keycode table

or

check them interactively here: http://keycode.info/ or http://keycodes.atjayjo.com/

The function String.fromCharCode will then convert a unicode number into a character.

For cross browser compatibility you might want to check both the e.which and e.keyCode properties like below.

function(e) {
  e = e || window.event;
  var keyCode= e.which || e.keyCode;
  var charStr = String.fromCharCode(keyCode);
  alert("The keycode is: "+ keyCode + " and charCode is: " + charStr);
}

You don't need to get the character for those key codes, you could just compare them directly:

function keyDown(e) {
  if (e.keyCode === 37) isLeft = true;
  if (e.keyCode === 39) isRight = true;
}

function keyUp(e) {
  if (e.keyCode === 37) isLeft = false;
  if (e.keyCode === 39) isRight = false;
}

Anyway, keep in mind e.keyCode is deprecated, so e.key or e.code should be used instead, which also makes the code easier to understand. Also, you could use an object with boolean properties instead of individual booleans to make this scale better to more keys:

const pressedKeys = {};

document.onkeydown = ({ key }) => {
  pressedKeys[key] = true;
  
  console.log(Object.keys(pressedKeys).join(' + '));
};

document.onkeyup = ({ key }) => {
  delete pressedKeys[key];
  
  console.log(Object.keys(pressedKeys).join(' + '));
};

Just keep in mind some old browsers used non-standard codes, so left is usually 'LeftArrow ' and right is 'RightArrow', but on IE it's just 'Left' and 'Right' instead.

Also, if you need to check KeyboardEvent's properties values such as e.key, e.code, e.which or e.keyCode you can use https://keyjs.dev. I will add information about these kinds of cross-browser incompatibilities soon!

Disclaimer: I'm the author.