Assign keyboard shortkeys to document.

keymap.ts

function noop(){};

function createKeydownListener(key: string, modifiers: string[] = [], cb: Function = noop) {
  const shouldHaveAlt: Boolean = modifiers.includes(keymap.ALT);
  const shouldHaveCtrl: Boolean = modifiers.includes(keymap.CTRL);
  const shouldHaveMeta: Boolean = modifiers.includes(keymap.META);
  const shouldHaveShift: Boolean = modifiers.includes(keymap.SHIFT);

  return function (event: KeyboardEvent) {
    if (event.keyCode !== key.toUpperCase().charCodeAt(0))
      return;

    if (shouldHaveAlt === event.altKey &&
        shouldHaveCtrl === event.ctrlKey &&
        shouldHaveMeta === event.metaKey &&
        shouldHaveShift === event.shiftKey) 
    {
      cb({key: key.toUpperCase(), modifiers, event}); 
    }
  }
}

function keymap(key: string, modifiers: string[] = [], cb: Function = noop) {
  if (key.length !== 1)
    throw new Error('key parameter should be a string of length 1');

  modifiers.forEach((modifier) => {
    if (! [keymap.ALT, keymap.CTRL, keymap.META, keymap.SHIFT].includes(modifier))
      throw new Error(`modifiers elements should be one of values '${keymap.ALT}', '${keymap.CTRL}', '${keymap.META}', '${keymap.SHIFT}'`)
  })

  document.body.addEventListener('keydown', createKeydownListener(key, modifiers, cb));
}

keymap.ALT = 'alt';
keymap.CTRL = 'ctrl'
keymap.META = 'meta';
keymap.SHIFT = 'shift';

keymap('a', ['ctrl', 'shift'], (x: Object) => {
  console.log(x)
});