<INPUT type=number > extra keys

input-number-extra-keys.js

const isMac = navigator.platform === 'MacIntel';

const KEY = {
  UP: 38,
  DOWN: 40,
};

document.querySelector("input").addEventListener("keydown", e => {
  if ([KEY.UP, KEY.DOWN].includes(e.keyCode)) {
    e.preventDefault();
    
    const currentValue = isNaN(parseFloat(e.target.value))
      ? parseFloat(e.target.getAttribute("min")) || 0
      : parseFloat(e.target.value);
      
    const direction = e.keyCode === KEY.UP ? 1 : -1;
    
    const modifier = (isMac ? e.metaKey : e.ctrlKey) ? 100 : e.shiftKey ? 10 : e.altKey ? 0.1 : 1;
    
    const decimals = Math.max(
      (currentValue.toString().split(".")[1] || "").length,
      e.altKey ? 1 : 0
    );
    
    const newValue = currentValue + direction * modifier;
    
    e.target.value = newValue.toFixed(decimals);
  }
});