pull data from form fields instantly?

How do I pull data from form fields instantly?

for example, timer = 1;

here's the form:

<form id="debugForm">
<label>Timer: </label><input type="text" name="timer" value="1" />;
</form>

How could I make it so that if the variable timer is set to 1, and I typed 10 in the field, then the timer variable would update instantly?