[j0hnik]

<input type="text">
<input type="text">
<button>1</button>
<button>2</button>
<script>
	var a;
	var inp = document.querySelectorAll('input');
	inp.forEach((el,i)=>el.onfocus = e => {
		if(el == e.target) a = i;
	});
	document.querySelectorAll('button').forEach(el=>el.onclick = e => inp[a].value += el.textContent);
</script>