Because I do most of my research while commuting by train, I often pull entire websites offline using httrack. While browsing the [jQuery Mobile documentation](http://jquerymobile.com/demos/1.1.0-rc.1/ locally this morning, I stumbled upon following gem.

I was curious to see how they determine whether a page is browsed locally or not. Looking into the source, I was a bit dissapointed to find nothing but plain common sense. The trick is comparing the protocol of the current location with known local protocols.

if ( location.protocol.substr(0,4)  === 'file' ||
        location.protocol.substr(0,11) === '*-extension' ||
        location.protocol.substr(0,6)  === 'widget' ) {
    // Disable AJAX support etc
}

If you would want to use that check in multiple locations in your codebase, you might want to extend the location object with an isLocal function.

window.location.constructor.prototype.isLocal = function() { 
    return this.protocol.substr(0,4)  === 'file' || 
            this.protocol.substr(0,11) === '*-extension' || 
            this.protocol.substr(0,6)  === 'widget'; 
}

The function could be used like this.

if (window.location.isLocal()) {
    // Disable AJAX support etc
}