We are making big changes in the way we build the Netflix experience with Facebook’s React library. Today, we will share our thoughts on what makes React so compelling and how it is evolving our approach to UI development.

At the beginning of last year, Netflix UI engineers embarked on several ambitious projects to dramatically transform the user experience on our desktop and mobile platforms. Given a UI redesign of a scale similar to that undergone by TVs and game consoles, it was essential for us to re-evaluate our existing UI technology stack and to determine whether to explore new solutions. Do we have the right building blocks to create best-in-class single-page web applications? And what specific problems are we looking to solve?

Much of our existing front-end infrastructure consists of hand-rolled components optimized for the current website and iOS application. Our decision to adopt React was influenced by a number of factors, most notably: 1) startup speed, 2) runtime performance, and 3) modularity.

Startup Speed

We want to reduce the initial load time needed to provide Netflix members with a much more seamless, dynamic way to browse and watch individualized content. However, we find that the cost to deliver and render the UI past login can be significant, especially on our mobile platforms where there is a lot more variability in network conditions.

In addition to the time required to bootstrap our single-page application (i.e. download and process initial markup, scripts, stylesheets), we need to fetch data, including movie and show recommendations, to create a personalized experience. While network latency tends to be our biggest bottleneck, another major factor affecting startup performance is in the creation of DOM elements based on the parsed JSON payload containing the recommendations. Is there a way to minimize the network requests and processing time needed to render the home screen? We are looking for a hybrid solution that will allow us to deliver above-the-fold static markup on first load via server-side rendering, thereby reducing the tax incurred in the aforementioned startup operations, and at the same time enable dynamic elements in the UI through client-side scripting.

Runtime Performance

To build our most visually-rich cinematic Netflix experience to date for the website and iOS platforms, efficient UI rendering is critical. While there are fewer hardware constraints on desktops (compared to TVs and set-top boxes), expensive operations can still compromise UI responsiveness. In particular, DOM manipulations that result in reflows and repaints are especially detrimental to user experience.

Modularity

Our front-end infrastructure must support the numerous A/B tests we run in terms of the ability to rapidly build out new features and designs that code-wise must co-exist with the control experience (against which the new experiences are tested). For example, we can have an A/B test that compares 9 different design variations in the UI, which could mean maintaining code for 10 views for the duration of the test. Upon completion of the test, it should be easy for us to productize the experience that performed the best for our members and clean up code for the 9 other views that did not.

Advantages of React

React stood out in that its defining features not only satisfied the criteria set forth above, but offered other advantages including being relatively easy to grasp and ability to opt-out, for example, to handle custom user interactions and rendering code. We were able to leverage the following features to improve our application’s initial load times, runtime performance, and overall scalability: 1) isomorphic JavaScript, 2) virtual DOM rendering, and 3) support for compositional design patterns.

Isomorphic JavaScript

React enabled us to build JavaScript UI code that can be executed in both server (e.g. Node.js) and client contexts. To improve our start up times, we built a hybrid application where the initial markup is rendered server-side and the resulting UI elements are subsequently manipulated as done in a single-page application. It was possible to achieve this with React as it can render without a live DOM, e.g. via React.renderToString, or React.renderToStaticMarkup. Furthermore, the UI code written using the React library that is responsible for generating the markup could be shared with the client to handle cases where re-rendering was necessary.

Virtual DOM

To reduce the penalties incurred by live DOM manipulation, React applies updates to a virtual DOM in pure JavaScript and then determines the minimal set of DOM operations necessary via a diff algorithm. The diffing of virtual DOM trees is fast relative to actual DOM modifications, especially using today’s increasingly efficient JavaScript engines such as WebKit’s Nitro with JIT compilation. Furthermore, we can eliminate the need for traditional data binding, which has its own performance implications and scalability challenges.

React Components and Mixins

React provides powerful Component and Mixin APIs that we relied on heavily to create reusable views, share common functionality, and patterns to facilitate feature extension. When A/B testing different designs, we can implement the views as separate React subcomponents that get rendered by a parent component depending on the user’s allocation in the test. Similarly, differences in behavioral logic can be abstracted into React mixins. Although it is possible to achieve modularity with a classical inheritance pattern, frequent changes in superclass interfaces to support new features affects existing subclasses and increases code fragility. React’s compositional pattern is ideal for overall maintenance and scalability of our front-end codebase as it isolates much of the A/B test code.

React has exceeded our requirements and enabled us to build a tremendous foundation on which to innovate the Netflix experience. Stay tuned in the coming months, as we will dive more deeply into how we are using React to transform traditional UI development!



— by Jordanna Kwok

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