Most of the web has transitioned into client-side heavy development using various JavaScript frameworks. More and more sites are being built in the “Single Page Application” (SPA) fashion. This puts more work on the user’s browser and less work on the servers. The shift keeps front-end and back-end development separate and isolated, allowing clients to present information in different ways without relying on server-side rendering. Additionally, most of smartphones these days are more powerful than servers that serve thousands of users.

[ Source Code | Demo ]

One of the most common issues with this paradigm is the slow initial load time of the page. Typically, the page is loaded very quickly only to show a spinning icon for some time until the actual content is loaded. Here’s the diagram that explains the request cycle:

  1. Client requests the page. Server responds with some bootstrapped and non-rendered HTML. (green)
  2. Client requests for static assets, including the JavaScript files and HTML templates. (blue)
  3. Application makes one or more HTTP calls to fetch various resources. (yellow)
  4. The loaded resources are displayed to the user.

These steps are sequential. As one can imagine, the initial load time can be very slow if the user has a slow internet connection and/or the user is far away from the website data centers.

By looking at the diagram, let’s assume the api/products/123 is the highest priority HTTP call. We want to show the result of it as soon as it is ready while other HTTP calls may still be pending. If we somehow include the response of this endpoint with the initial HTML page or app.js, the extra HTTP call would not be necessary.

AngularJS and Angular UI Router

AngularJS is a popular and somewhat opinionated JavaScript framework that is backed by Google. (Fun fact: the Angular team sat one floor below my team at Google!)

AngularJS lets you write client-side web applications as if you had a smarter browser. It lets you use good old HTML (or HAML, Jade and friends!) as your template language and lets you extend HTML’s syntax to express your application’s components clearly and succinctly. It automatically synchronizes data from your UI (view) with your JavaScript objects (model) through 2-way data binding.

If you are building an AngularJS 1.x app, you probably are already familiar with AngularUI Router. It is generally more flexible than the standard $route service. I suggest looking into it if you have not done so. The example in this post uses AngularUI Router.

A typical application route setup might look something like this:

    .state('main', {
        abstract: true,
        resolve: {
            user: function (UserService) {
    .state('main.product', {
        url: '/products/{id}'
        resolve: {
            product: function ($stateParams, ItemService) {
                return ItemService.get($;

The user and product properties are “resolved” (by making HTTP calls for example) before the state transition happens.

Preload (Prefetch) Data

The solution I propose is simple but somewhat ugly to implement. It minimizes the initial load time, which is the best user experience.

  1. When the server receives a request to, it makes an API call to api/products/123 on behalf of the client. This will be significantly faster than the client calling the API endpoint because the data centers are typically close to each other with much faster network speeds.
  2. The result from api/products/123 is appended to the original as URL parameter (let’s call it _data). The URL change to:{...}
  3. The client handles the case when this URL parameter is present.

Important: In order for the proposed solution to work, the application needs to have html5Mode enabled. This is because anything after the # portion of the URL is not sent to the server (read more: fragment identifier). The default hashbang method uses the #. You should have strong reasons if you do not already have html5Mode enabled.

In this scenario, we are only working with api/products/123. The solution can be used in 100 different ways, including but not limited to calling multiple APIs or including some placeholder data.

First, let’s make that URL parameter an optional parameter (_data).

class Config {
    constructor($locationProvider, $stateProvider, $urlRouterProvider) {


            .state('default', {
                url: '/',
                templateUrl: 'main.html'
            .state('item', {
                url: '/items/{id}?_data',
                controller: 'Item as item',
                templateUrl: 'item.html',
                reloadOnSearch: false

It makes sense to remove the _data parameter from the URL as soon as the application reads so the user does not share or bookmark the full URL (and to make the URL look pretty). The reloadOnSearch: false option prevents the state from being reloaded when this happens.

Now, the client logic skips the API call to get product information when the _data parameter is present.

class Item {
    constructor($log, $state, $stateParams, ItemService) {
        this.title = 'Loading..';
        this._log = $log;
        this._state = $state;
        this._stateParams = $stateParams;
        this._ItemService = ItemService;


    init() {
        let context = this;


        // Pre-fetched data can come as a URL parameter (`_data`).
        var data = angular.fromJson(this._stateParams._data);

        if (data) {
            // Remove `_data` parameter from URL.
            this._state.go('.', {_data: null}, {location: 'replace'});

            return ready(data);

        this._ItemService.fetchItem(, ready);

        function ready(data) {
            context.title =;
            context.item = data;

class ItemService {
    constructor($timeout) {
        this._timeout = $timeout;

    fetchItem(id) {
      return this._timeout(() => {
          return {id: id, name: 'TV', description: 'Fetched from the backend'};
      }, 1500);

This method reduces the number of HTTP requests made by the application. However, I do not recommend doing this everywhere in the application as it introduces logic code both in the back-end endpoint and in the Angular application.

Full source and demo can be found here.


We have looked at one of the biggest concerns against modern client-side heavy development – slower initial load times. We proposed a back-end agnostic solution for the problem in AngularJS that makes use of URL parameters. Code samples for both front-end and back-end have been provided.

Bonus: Flask Back-end Endpoint Handler

Below is the quick implementation of step-1 and step-2 of the proposed solution above, in Flask. I’m sure a similar approach can be implemented in other frameworks.

    from urllib.parse import urlencode
    from urllib import urlencode

import re
import flask as f

app = f.Flask(__name__)
ITEMS_RE = re.compile(r'^items/(\d+)')

@app.route('/app/', defaults={'path': ''})
def index(path):
    """Main application entry. Let's assume our app is served at `/app`."""
    m = ITEMS_RE.match(path)
    if m and not f.request.args.get('_data'):
        data = get_item(int(m.groups()[0]))
        args = dict(f.request.args)
        args['_data'] = f.json.dumps(data)
        url = '{}?{}'.format(f.request.base_url, urlencode(args, doseq=True))
        return f.redirect(url)
    return 'return index.html'

def get_item(ident):
    return {'id': ident, 'name': 'fake', 'description': 'fake api return'}

if __name__ == '__main__':