A Google Chrome Extensions helper class for accessing page context (DOM manipulation, etc.) from the background. Use a node.js-style callback pattern to create a custom page API.

background.js

(function(){
    
    chrome.tabs.getSelected(null, function(tab){
        
        var page = new Page(tab);
        
        page.setScrollTo(100, 200, function(err, res){
            
            console.log('You just scrolled the page!');
            
        });
        
        // Or using `async`...
        
        async.parallel([
            async.apply(page.setScrollTo, 100, 200),
            page.getDocumentSize
        ], function(err, res){
            
            console.log('You just scrolled the page AND got the document size!');
            
        });
        
    });
    
})();

manifest.json

{
  "manifest_version": 2,
  "background": {
    "scripts": [
      "underscore.js",
      "async.js",
      "page.js",
      "page-api.js",
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "underscore.js",
        "page.js",
        "page-api.js"
      ]
    } 
  ],
  "permissions": [
    "http://*/*",
    "https://*/*",
    "tabs"
  ]
}

page-api.js

(function(){
    
    _.extend(Page.prototype, {
        
        // Customize this file with your page-specific methods, e.g.
        
        setScrollTo: function (left, top, callback) {
            window.scrollTo(left, top);
            callback(null);
        },
        
        getDocumentSize: function (callback) {
            callback(null, {
                width: document.body.scrollWidth,
                height: document.body.scrollHeight
            });
        }
        
    });
    
})();

page.js

(function(ctx, isPage){
    
    var send = function (id, name, args, callback) {
        chrome.tabs.sendMessage(id, {
            name: name,
            args: args
        }, function(response){
            if (response.error)
            {
                return callback(response.error);
            }
            callback.apply(null, response.args);
        });
    };
    
    var Page = function (tab) {
        
        var self = this;
        
        _.bindAll(self);
        
        if (isPage)
        {
            chrome.extension.onMessage.addListener(function(message, sender, callback){
                
                var method = self[message.name];
                
                if (!_.isFunction(method))
                {
                    return callback({
                        error: 'Method' + message.name + 'does not exist'
                    });
                }
                
                message.args.push(function(){
                    callback({
                        error: null,
                        args: _.toArray(arguments)
                    });
                });
                
                method.apply(null, message.args);
                
            });
        }
        else
        {
            if (!tab)
            {
                throw new Error('Must reference a tab');
            }
            
            _.forEach(Page.prototype, function(val, key){
                
                self[key] = function () {
                    
                    var args = _.toArray(arguments),
                        callback = args.pop();
                    
                    send(tab.id, key, args, callback);
                };
                
            });
        }
    };
    
    if (isPage)
    {
        new Page();
    }
    
    ctx.Page = Page;
    
})(this, (this.outerWidth > 0 && this.outerHeight > 0));