oninput

js/script.js

function funRes() {
var val0 = document.getElementById('val0');
var val1 = document.getElementById('val1');
var res = document.getElementById('res');
//false, null, '', "", !val1.value
if(val0.value == false) {
val0.value = 0;
}
if(!val1.value) {
val1.value = 0;
}
res.value = parseFloat(val0.value) + parseFloat(val1.value);
}
function funEmpty0() {
var val0 = document.getElementById('val0');
val0.value = '';
}
function funEmpty1() {
var val1 = document.getElementById('val1');
val1.value = '';
}
function funReset() {
var val0 = document.getElementById('val0');
var val1 = document.getElementById('val1');
var res = document.getElementById('res');
val0.value = '';
val1.value = '';
res.value = '';
}
function funCSS() {
document.getElementById('reset').onclick = funReset;
var ara = document.getElementsByTagName('input');
for(var i=0; i<ara.length; i++) {
ara[i].style.width = '100px';
ara[i].style.textAlign = 'center';
}
var val = document.getElementById('main');
val.style.height = '100px';
val.style.width = '600px';
val.style.position = 'absolute';
val.style.top = '50%';
val.style.left = '50%';
val.style.marginTop = '-50px';
val.style.marginLeft = '-300px';
val.style.textAlign = 'center';
val.style.lineHeight = '100px';
}
window.onload = function() {
document.getElementById('val0').oninput = funRes;
document.getElementById('val1').oninput = funRes;
document.getElementById('val0').onfocus = funEmpty0;
document.getElementById('val1').onfocus = funEmpty1;
funCSS();
}

function funRes() {
var val0 = document.getElementById('val0');
var val1 = document.getElementById('val1');
var res = document.getElementById('res');
//false, null, '', "", !val1.value
if(val0.value == false) {
val0.value = 0;
}
if(!val1.value) {
val1.value = 0;
}
res.value = parseFloat(val0.value) + parseFloat(val1.value);
}
function funEmpty0() {
var val0 = document.getElementById('val0');
val0.value = '';
}
function funEmpty1() {
var val1 = document.getElementById('val1');
val1.value = '';
}
function funReset() {
var val0 = document.getElementById('val0');
var val1 = document.getElementById('val1');
var res = document.getElementById('res');
val0.value = '';
val1.value = '';
res.value = '';
}
function funCSS() {
document.getElementById('reset').onclick = funReset;
var ara = document.getElementsByTagName('input');
for(var i=0; i<ara.length; i++) {
ara[i].style.width = '100px';
ara[i].style.textAlign = 'center';
}
var val = document.getElementById('main');
val.style.height = '100px';
val.style.width = '600px';
val.style.position = 'absolute';
val.style.top = '50%';
val.style.left = '50%';
val.style.marginTop = '-50px';
val.style.marginLeft = '-300px';
val.style.textAlign = 'center';
val.style.lineHeight = '100px';
}
window.onload = function() {
document.getElementById('val0').oninput = funRes;
document.getElementById('val1').oninput = funRes;
document.getElementById('val0').onfocus = funEmpty0;
document.getElementById('val1').onfocus = funEmpty1;
funCSS();
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<div id="main">
<input id="val0" type="text" />
  
<label>+</label>
  
<input id="val1" type="text" />
  
<label>=</label>
  
<input id="res" type="text" readonly="readonly" />
     
<input id="reset" type="button" value="Reset" />
</div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<div id="main">
<input id="val0" type="text" />
  
<label>+</label>
  
<input id="val1" type="text" />
  
<label>=</label>
  
<input id="res" type="text" readonly="readonly" />
     
<input id="reset" type="button" value="Reset" />
</div>
</body>
</html>