measure time since last keypress

HTML

<input id="txt" placeholder="type something here" style="width:100%" />
<p>Time since previous keypress:
<div id="output"></div>
<p>Mean time between keypresses:
<div id="mean">
</div>

JavaScript

var last;
var output = $('#output');
var mean = $('#mean');
var interval = [];
$('#txt').on('input', function() {
    var n = new Date()
    output.text((n - last) + ' ms');
    interval.push(n - last);
    
    var sum = 0;
		for( var i = 0; i < interval.length; i++ ){
    		sum += parseInt(interval[i], 10); //don't forget to add the base
		}
		var avg = sum/interval.length;
    
    mean.text(avg + 'ms');
    
    last = n;
});