Prevent iOS form element focus zoom

// ======================================
// iOS zooms on form element focus. 
// This script prevents that behavior.
//
// <meta name="viewport" content="width=device-width,initial-scale=1">
// If you dynamically add a maximum-scale where no default exists,
// the value persists on the page even after removed from viewport.content.
// So if no maximum-scale is set, adds maximum-scale=10 on blur.
// If maximum-scale is set, reuses that original value.
// 
// <meta name="viewport" content="width=device-width,initial-scale=1.0,
                                  maximum-scale=2.0,maximum-scale=1.0">
//      second maximum-scale declaration will take precedence.
// 
// - Will respect original maximum-scale, if set.
// - Works with int or float scale values.
// ======================================



function cancelZoom(){
    var d = document,
        viewport,
        content,
        maxScale = ',maximum-scale=',
        maxScaleRegex = /,*maximum\-scale\=\d*\.*\d*/;

    // this should be a focusable DOM Element
    if(!this.addEventListener || !d.querySelector) { return; }

    viewport = d.querySelector('meta[name="viewport"]');
    content = viewport.content;

    function changeViewport(event) {
        // http://nerd.vasilis.nl/prevent-ios-from-zooming-onfocus/
        viewport.content = content + (event.type == 'blur' ? (content.match(maxScaleRegex, '') ? '' : maxScale + 10) : maxScale + 1);
    }

    // We could use DOMFocusIn here, but it's deprecated.
    this.addEventListener('focus', changeViewport, true);
    this.addEventListener('blur', changeViewport, false);
}

// jQuery-plugin
(function($){
    $.fn.cancelZoom = function() { return this.each(cancelZoom); };
    // Usage:
    $('input:text,select,textarea').cancelZoom();
})(jQuery);