Returns an object whose keys are keyboard key names and whose values are booleans indicating whether the key is pressed or not.

watchKeys.js

function watchKeys(target = document) {
	const watcher = {};

	target.addEventListener('keydown', function (e) {
		watcher[e.key] = true;
	});

	target.addEventListener('keyup', function (e) {
		watcher[e.key] = false;
	});

	return watcher;
}

// EXAMPLE:
const watcher = watchKeys();

const player = {
	x: 0,
	y: 0
};

const ctx = document.createElement('canvas').getContext('2d');
document.body.appendChild(ctx.canvas);

setInterval(function () {
	if (watcher.ArrowLeft) {
		player.x--;
	} else if (watcher.ArrowRight) {
		player.x++;
	}

	if (watcher.ArrowUp) {
		player.y--;
	} else if (watcher.ArrowDown) {
		player.y++;
	}
	
	ctx.fillRect(player.x, player.y, 1, 1);
}, 1000 / 30);