Event Handler That Clears Text Fields

I am having a dickens of a time figuring out how to add an event handler that clears my text fields when the clear button is clicked. This is the code that I've done but to no avail.

var $ = function (id) {
return document.getElementById(id);}
var calculate_click = function () {
var investment = parseFloat( $("investment").value );
var annualRate = parseFloat( $("rate").value );
var years = parseInt( $("years").value );
$("futureValue").value = "";
if (isNaN(investment) || investment <= 0) {
alert("Investment must be a valid number\nand greater than zero.");
} else if(isNaN(annualRate) || annualRate > 20) {
alert("Annual rate must be a valid number\nand less than or equal to 20.");
} else if(isNaN(years) || years > 50) {
alert("Years must be a valid number\nand less than or equal to 50.");
} else {
var monthlyRate = annualRate / 12 / 100;
var months = years * 12;
var futureValue = 0;

for ( i = 1; i <= months; i++ ) {
futureValue = ( futureValue + investment ) *
(1 + monthlyRate);
}
$("futureValue").value = futureValue.toFixed(2);
}}
var clear_click = function () {
var investment = parseFloat();
var annualRate = parseFloat();
var years = parseInt();
}
window.onload = function () {
$("calculate").onclick = calculate_click;
$("investment").focus();
}