Script streaming optimizes the parsing of JavaScript files. Previous versions of Chrome would download a script in full before beginning to parse it, which is a straightforward approach but doesn't fully utilize the CPU while waiting for the download to complete. Starting in version 41, Chrome parses async and deferred scripts on a separate thread as soon as the download has begun. This means that parsing can complete just milliseconds after the download has finished, and results in pages loading as much as 10% faster. It's particularly effective on large scripts and slow network connections.
Code caching is another new technique that helps speed up page loading, specifically on repeated visits to the same page. Normally, the V8 engine compiles the page’s JavaScript on every visit, turning it into instructions that a processor understands. This compiled code is then discarded once a user navigates away from the page as compiled code is highly dependent on the state and context of the machine at compilation time. Chrome 42 introduces an advanced technique of storing a local copy of the compiled code, so that when the user returns to the page the downloading, parsing, and compiling steps can all be skipped. Across all page loads, this allows Chrome to avoid about 40% of compile time and saves precious battery on mobile devices.
These are two examples of ways Chrome is improving page load time, but page load time is just one way to think about the performance of the browser. Stay tuned for more ways the Chromium project is pushing forward all aspects of performance on the web.
This release includes two new APIs that together allow sites to push native notifications to their users even after the page is closed—provided the user has granted explicit permission. After the user has granted permission, a developer can use the new Push API to remotely wake up their service worker using Google Cloud Messaging. Once awake, the service worker may run JavaScript for a short period but in this release it is required at minimum to show a user-visible notification. Each notification includes a ‘Site Settings’ button, allowing users to easily disable notifications for a site.
Promoting Add to Home Screen
Chrome 32 introduced the ability for Android users to add home screen shortcuts to their favorite websites via a menu item. In this release, users who frequently visit a high-quality web app will see a banner that allows them to add the site to their home screen in one tap.
Sites that wish to take advantage of this new feature must meet eligibility criteria that ensure that users have a good experience when launching sites from the home screen, even when offline. The criteria will evolve over time based on feedback from users and developers, but today they require sites to provide a Web App Manifest, serve all content using HTTPS, and at least partially work offline using a service worker.
ES6 Classes
Developers often find it hard to adapt to JavaScript’s prototype-based inheritance and although many libraries have introduced their own patterns for emulating classes, the language hasn't yet provided a single, uniform way of describing them.
ES6 classes solve this by providing JavaScript a clean, standardized syntax for classes. This new syntax is available in Chrome 42 for JavaScript written in strict mode.
'use strict';
class Polygon {
constructor(height, width) {
this.name = 'Polygon';
this.height = height;
this.width = width;
}
sayName() {
log('Hi, I am a ', this.name + '.');
}
}
let p = new Polygon(300, 400);
Other updates in this release
DevTools now allows developers to visually edit cubic beziers directly from the styles pane, making it easier to understand and modify animations.
The Fetch API is now available in the window context, shared workers, and dedicated workers, providing a new promise-based standard for AJAX requests.
AudioBufferSourceNode.buffer can no longer be set more than once, protecting developers from the lack of control over when the new source starts.
Chrome OS now supports screen.orientation and fires the DeviceOrientationEvent when the device’s orientation changes significantly, allowing orientation-aware websites to operate correctly on Chrome OS devices.
This release includes an updated and unprefixed implementation of Encrypted Media Extensions, which allows media sites to discover and interact with digital rights management systems..
A new content setting allows users to automatically pause non-primary plugin content to save power. Developers can turn it on to test how it interacts with their content.
