JavaScript: Field Multiplier

field_multiplier.js

var FieldMultiplier = {
	makeSum: function(e) {
		/**
		 * Look for any elements with the same class as this one
		 * and sum them, multiplying by the multiplier attribute.
		 * The sum will go into the elements that match the sum attribute.
		 */
		var sum = 0;
		var sum_selector = '.'+e.getAttribute('class');
		var elements = document.querySelectorAll(sum_selector);
		var length = elements.length;
		for(var i = 0; i < length; ++i) {
			var multiplier = elements[i].getAttribute('multiplier');
			var value = parseInt(elements[i].value);
			if(value) {
				if(multiplier) {
					value *= multiplier;
				}
				sum += value;
			}
		}
		var target_selector = e.getAttribute('sum');
		var target_elements = document.querySelectorAll(target_selector);
		var target_length = target_elements.length;
		for(var j = 0; j < target_length; ++j) {
			target_elements[j].innerHTML = sum.toFixed(2);
		}
	}
};