JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id="btnDown" value="-">
<span id="amountDiv">amount div</span>
<input type="button" id="btnUp" value="+">

JavaScript

var testNum = 20;

amountDiv.innerHTML = testNum;

function makeButtonIncrement(button, upDown, displayDiv, variable) {
	
	function changeValue () {
		if (upDown == "up") {
			variable++;
		}
		
		if (upDown == "down") {
			variable--;
		}
		
		displayDiv.innerHTML = variable;
	}
	
	button.onclick = changeValue;
	
	return variable;
}

makeButtonIncrement(document.getElementById('btnDown'), "down", document.getElementById('amountDiv'), testNum);

makeButtonIncrement(document.getElementById('btnUp'), "up", document.getElementById('amountDiv'), testNum);