Simplest way to detect a pinch

Think about what a pinch event is: two fingers on an element, moving toward or away from each other.Gesture events are, to my knowledge, a fairly new standard, so probably the safest way to go about this is to use touch events like so:

(ontouchstart event)

if (e.touches.length === 2) {    scaling = true;    pinchStart(e);}

(ontouchmove event)

if (scaling) {    pinchMove(e);}

(ontouchend event)

if (scaling) {    pinchEnd(e);    scaling = false;}

To get the distance between the two fingers, use the hypot function:

var dist = Math.hypot(    e.touches[0].pageX - e.touches[1].pageX,    e.touches[0].pageY - e.touches[1].pageY);

You want to use the gesturestart, gesturechange, and gestureend events. These get triggered any time 2 or more fingers touch the screen.

Depending on what you need to do with the pinch gesture, your approach will need to be adjusted. The scale multiplier can be examined to determine how dramatic the user's pinch gesture was. See Apple's TouchEvent documentation for details about how the scale property will behave.

node.addEventListener('gestureend', function(e) {    if (e.scale < 1.0) {        // User moved fingers closer together    } else if (e.scale > 1.0) {        // User moved fingers further apart    }}, false);

You could also intercept the gesturechange event to detect a pinch as it happens if you need it to make your app feel more responsive.

Hammer.js all the way! It handles "transforms" (pinches).http://eightmedia.github.com/hammer.js/

But if you wish to implement it youself, i think that Jeffrey's answer is pretty solid.