SO Adder

adder.html

<script>
  var inputs = {};
	function save(value, name) {
		inputs[name] = parseInt(value);
	}
	function checker(criterion, action) {
		return function(value, name) {
			if( !criterion(value) ) {
				alert("Not Numeric");
			} else {
				action(value, name);
			}
		};
	}
	function isNumber(x) {
		return !isNaN(x);
	}
	function addFunction(elm) {
		elm.value = inputs.num1 + inputs.num2;
	}
</script>	

<input id="num1" type="text" onblur="checker(isNumber, save)(this.value, 'num1')">+

<input id="num2" type="text" onblur="checker(isNumber, save)(this.value, 'num2')"> = <input id="adds" type="text" onfocus="addFunction(this)">