User-Agent reduction snippets

A collection of code snippets to let you transform the current Chrome user-agent string to the new reduced format. Submit versions for your preferred platforms and languages or let us know what would be useful to add!

# Background

Chrome is reducing the information exposed in its user-agent string. This will be happening incrementally over a period of time with the final state removing the OS version, device, and full browser version portions of the string.

Here's an example of how that would look on a mobile device:

# How to override the user-agent in your code

You can test the string locally with the help of regular expressions.

As the format for the reduced user-agent string is available, this means you can transform and test the new string against your own code—either by overriding and replacing it, or by generating the new version and testing side-by-side.

# Regular expression

A regular expression that will only match safely transformable Chrome user-agents and pulls out the necessary variable values to slot into a template for the new format.

All on one line for easy copying. It is long, but that's to prioritise only matching against Chrome and doing with relatively cheap checks.

/^Mozilla\/5\.0 \(((?Lin|Win|Mac|X11; C|X11; L)+[^\)]+)\) AppleWebKit\/537.36 \(KHTML, like Gecko\) Chrome\/(?\d+)[\d\.]+(?[ Mobile]*) Safari\/537\.36$/

The expression captures the following values:

• platform: captures the platform or operating system
• major: captures the browser major version
• mobile: captures the mobile device indicator

The captured values should be inserted into this template string for the reduced format.

Mozilla/5.0 (${unifiedPlatform[matched.platform]}) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/${matched.major}.0.0.0${matched.mobile} Safari/537.36

The unifiedPlatform value comes from taking the short platform match and replacing it with the appropriate reduced value.

• Lin → Linux; Android 10; K
• Win → Windows NT 10.0; Win64; x64
• Mac → Macintosh; Intel Mac OS X 10_15_7
• X11; C → X11; CrOS x86_64 14541.0.0
• X11; L → X11; Linux x86_64

# JavaScript

In client-side JavaScript, the following snippet can be used to override the current value of navigator.userAgent with the reduced format.

Demo: reduced-ua.glitch.me/javascript.html

const chromeUAs = /^Mozilla\/5\.0 \(((?<platform>Lin|Win|Mac|X11; C|X11; L)+[^\)]+)\) AppleWebKit\/537.36 \(KHTML, like Gecko\) Chrome\/(?<major>\d+)[\d\.]+(?<mobile>[ Mobile]*) Safari\/537\.36$/;
const matched = chromeUAs.exec(navigator.userAgent);

if (matched) {
  // Map detected platform to reduced value
  const unifiedPlatform = {
    'Lin': 'Linux; Android 10; K',
    'Win': 'Windows NT 10.0; Win64; x64',
    'Mac': 'Macintosh; Intel Mac OS X 10_15_7',
    'X11; C': 'X11; CrOS x86_64 14541.0.0',
    'X11; L': 'X11; Linux x86_64',
  };
  const reducedUA =
        `Mozilla/5.0 (${unifiedPlatform[matched.groups.platform]}) ` +
        `AppleWebKit/537.36 (KHTML, like Gecko) ` +
        `Chrome/${matched.groups.major}.0.0.0${matched.groups.mobile} Safari/537.36`
  // Override navigator.userAgent with the reduced string
  Object.defineProperty(navigator, 'userAgent', {
    value: reducedUA,
    writable: false,
    configurable: true
  });
}

# Node.js

In Node.js you can use the following snippet to override the incoming User-Agent header with the reduced format.

Demo: reduced-ua.glitch.me/server-side

const chromeUAs = /^Mozilla\/5\.0 \(((?<platform>Lin|Win|Mac|X11; C|X11; L)+[^\)]+)\) AppleWebKit\/537.36 \(KHTML, like Gecko\) Chrome\/(?<major>\d+)[\d\.]+(?<mobile>[ Mobile]*) Safari\/537\.36$/;
const matched = chromeUAs.exec(request.get('user-agent'));

if (matched) {
  const unifiedPlatform = {
    'Lin': 'Linux; Android 10; K',
    'Win': 'Windows NT 10.0; Win64; x64',
    'Mac': 'Macintosh; Intel Mac OS X 10_15_7',
    'X11; C': 'X11; CrOS x86_64 14541.0.0',
    'X11; L': 'X11; Linux x86_64',
  };
  request.headers['user-agent'] = `Mozilla/5.0 (${unifiedPlatform[matched.groups.platform]}) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/${matched.groups.major}.0.0.0${matched.groups.mobile} Safari/537.36`;
}

# CloudFlare Workers

You can use a CloudFlare Worker to proxy and transform requests to your own site. This snippet replaces the incoming User-Agent header with the reduced format.

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
})

async function handleRequest(request) {
  const chromeUAs = /^Mozilla\/5\.0 \(((?<platform>Lin|Win|Mac|X11; C|X11; L)+[^\)]+)\) AppleWebKit\/537.36 \(KHTML, like Gecko\) Chrome\/(?<major>\d+)[\d\.]+(?<mobile>[ Mobile]*) Safari\/537\.36$/;
  const matched = chromeUAs.exec(request.headers.get('user-agent'));

  if (matched) {
    const unifiedPlatform = {
      'Lin': 'Linux; Android 10; K',
      'Win': 'Windows NT 10.0; Win64; x64',
      'Mac': 'Macintosh; Intel Mac OS X 10_15_7',
      'X11; C': 'X11; CrOS x86_64 14541.0.0',
      'X11; L': 'X11; Linux x86_64',
    };

    const clonedRequest = new Request(request, {referrer: request.referrer});
    clonedRequest.headers.set('user-agent', `Mozilla/5.0 (${unifiedPlatform[matched.groups.platform]}) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/${matched.groups.major}.0.0.0${matched.groups.mobile} Safari/537.36`);
    return await fetch(clonedRequest);
  } else {
    return await fetch(request);
  }
}