Pinch to zoom using Hammer.js

For hammer.js 2.0+ I have taken @DGS answer and changed it to suit what I was doing with cordova and intel-xdk so it’s pure JS and hammer.js 2.0 for webkit.

My implementation allows you to zoom and drag at the same time, not independent of each other as above, and provides for a more native feel. It also implements the double tap to zoom in (and to zoom back out). I have it set to zoom between .999 and 4, but you can do as you like just changing those values. So if you just copy and paste this it will probably do what you expect it to (on webkit).

Thanks to Eight Media and @DGS for getting me started! Feel free to improve it SO.

function hammerIt(elm) {
    hammertime = new Hammer(elm, {});
    hammertime.get('pinch').set({
        enable: true
    });
    var posX = 0,
        posY = 0,
        scale = 1,
        last_scale = 1,
        last_posX = 0,
        last_posY = 0,
        max_pos_x = 0,
        max_pos_y = 0,
        transform = "",
        el = elm;

    hammertime.on('doubletap pan pinch panend pinchend', function(ev) {
        if (ev.type == "doubletap") {
            transform =
                "translate3d(0, 0, 0) " +
                "scale3d(2, 2, 1) ";
            scale = 2;
            last_scale = 2;
            try {
                if (window.getComputedStyle(el, null).getPropertyValue('-webkit-transform').toString() != "matrix(1, 0, 0, 1, 0, 0)") {
                    transform =
                        "translate3d(0, 0, 0) " +
                        "scale3d(1, 1, 1) ";
                    scale = 1;
                    last_scale = 1;
                }
            } catch (err) {}
            el.style.webkitTransform = transform;
            transform = "";
        }

        //pan    
        if (scale != 1) {
            posX = last_posX + ev.deltaX;
            posY = last_posY + ev.deltaY;
            max_pos_x = Math.ceil((scale - 1) * el.clientWidth / 2);
            max_pos_y = Math.ceil((scale - 1) * el.clientHeight / 2);
            if (posX > max_pos_x) {
                posX = max_pos_x;
            }
            if (posX < -max_pos_x) {
                posX = -max_pos_x;
            }
            if (posY > max_pos_y) {
                posY = max_pos_y;
            }
            if (posY < -max_pos_y) {
                posY = -max_pos_y;
            }
        }


        //pinch
        if (ev.type == "pinch") {
            scale = Math.max(.999, Math.min(last_scale * (ev.scale), 4));
        }
        if(ev.type == "pinchend"){last_scale = scale;}

        //panend
        if(ev.type == "panend"){
            last_posX = posX < max_pos_x ? posX : max_pos_x;
            last_posY = posY < max_pos_y ? posY : max_pos_y;
        }

        if (scale != 1) {
            transform =
                "translate3d(" + posX + "px," + posY + "px, 0) " +
                "scale3d(" + scale + ", " + scale + ", 1)";
        }

        if (transform) {
            el.style.webkitTransform = transform;
        }
    });
}

To implement just call it with hammerIt(document.getElementById(“imagid”)); after the element has loaded. You can call this on as many elements as you like.

Cheers!