Client Hints

Module provides various parameters from the client device, e.g. type of the device, screen size, etc.

Installation

First, install @tramvai/module-client-hints

npm i --save @tramvai/module-client-hints

Then add ClientHintsModule to the modules list:

import { createApp } from '@tramvai/core';
import { ClientHintsModule } from '@tramvai/module-client-hints';

createApp({
  modules: [ClientHintsModule],
});

It will enable server side user agent parsing. If you are using CSR fallback feature, then you should use ClientHintsCSRModule from this package instead.

Explanation

The problem with media on server and on client

One of the SSR problem is render of the component which depends on current screen size, e.g. image carousel that should render specific number of images depending on screen width. By default, the exact screen size can be figured out only on client-side and we can't render such content on server identical to the client render. If this content is not important for the SEO we can use skeletons and spinners, but they are not suitable for every case.

Client Hints modules provides the way to solve this problem in some way. It stores data about client devices in cookies and then use these cookies on server in next page loading.

Server-side rendering

Module will parse client hints/user agent only on the server by default. Parsing is implemented with library @tinkoff/user-agent that may use either user-agent header or client-hints headers.

If there is a sec-ch-ua header in request than user agent parsing will be based on Client Hints headers. If there is no such header than old school parsing of user-agent string will be used.

This logic implies next things worth to mention:

• by default, only part of client-hints is sent by browser and you can get only partial info about user browser (no cpu spec, platform version or device model). Although, we send an additional header accept-ch with response from server to request this data from client - on first request from current browser there will be no such data in any case and they will appear only on subsequent requests
• if you need to use additional info, you may specify the header accept-ch in your app with REQUEST_MANAGER_TOKEN
• client-hints is mostly more performant way to parse browser info and this is way it used if it's possible
• currently only chromium based browsers support client hints, so for other browsers and bots user-agent header will be used to gather browser info

Client-side rendering

If you want to parse user agent on the client, then you should use ClientHintsCSRModule:

import { createApp } from '@tramvai/core';
import { ClientHintsCSRModule } from '@tramvai/module-client-hints';

createApp({
  modules: [ClientHintsCSRModule],
  // Also, there will be no conflict with ClientHintsModule, but ClientHintsCSRModule must be registered after ClientHintsModule strictly.
  // modules: [ClientHintsModule, ClientHintsCSRModule],
});

:::warning

Usage of ClientHintsCSRModule will increase bundle size for ~ 18kb raw and 8kb gzip

:::

How does media work

First page loading

:::warning

When user enters the app for the first time, information about real device screen size and type not available in server-side code.

:::

This module tries to determine type of the user device using user-agent string, and separates the devices into three groups:

• mobile
• tablet
• desktop

Then it saves this assumptive information about device screen to media store. E.g. when user loads page from the desktop, then content of the media store will be following:

const state = {
  // desktop - 1024px, tablet - 600px, mobile - 300px
  width: 1024,
  // desktop - 768px, tablet - 800px, mobile - 500px
  height: 768,
  // desktop - false, tablet - true, mobile - true
  isTouch: false,
  retina: false,
  displayMode: 'browser',
  supposed: true,
  synchronized: false,
};

On the client focusing on value supposed: true module resolves real info about client device, updates media store and calls the rerender for the dependent components. E.g. for the widescreen monitor the data of media store might be next:

const state = {
  width: 1920,
  height: 1080,
  isTouch: false,
  retina: true,
  displayMode: 'browser',
  supposed: false,
  synchronized: false,
};

While we have value synchronized: false it is not allowed to use data from the media store for on the server-side as data is not synchronized with the client and it will lead to page jumps when saving real data about device.

Next page loads

When user loads the app next time the data about user device will be read from cookies and value synchronized will be set to true. This way on server and on client we will get the same content of the media store and no page rerenders on the client:

const state = {
  width: 1920,
  height: 1080,
  isTouch: false,
  retina: true,
  displayMode: 'browser',
  supposed: false,
  synchronized: true,
};

Use ClientHints in component

If some component depends on the screen size:

1. When user loads app for the first time is not possible to guarantee the same exact render on server and client
2. On first app load you may show some skeleton to the user by checking supposed: true property
3. You can guarantee the same exact render on server and client only in case synchronized: true

Api

Stores

userAgent

Stores the result of the user-agent string or client-hints headers parsing.

media

Stores the media information about type and size of the client screen.

media helpers

media store has next data:

type Media = {
  width: number;
  height: number;
  isTouch: boolean;
  retina: boolean;
  displayMode: 'browser' | 'standalone' | 'unknown';
  supposed?: boolean;
  synchronized?: boolean;
};

fromClientHints(media: Media): boolean - returns true if media data is synchronized on client and server

isSupposed(media: Media): boolean - returns true if media data are determined on server by the user-agent string and will be changes on the client

isRetina(media: Media): boolean - returns true if pixel density is equal to 2 or higher

useMedia(): Media - returns current state of the media store

useFromClientHints(): boolean - calculates fromClientHints

useIsSupposed(): boolean - calculates isSupposed

useIsRetina(): boolean - calculates isRetina

useDisplayMode(): boolean - calculates displayMode. It indicates the mode in which the application is opened. The possible values are:

• browser: The application is opened in a browser
• standalone: The application is opened as a Progressive Web App (PWA)
• unknown: The application mode could not be determined

How to

Render skeleton only when user loads pages first time

const App = () => {
  const isSupposed = useIsSupposed();

  if (isSupposed) {
    return <AdaptiveSliderSkeleton />;
  }

  return <AdaptiveSlider />;
};

Render adaptive component on first time and on subsequent loads render specific component

const App = () => {
  const media = useMedia();
  const fromClientHints = useFromClientHints();

  let Block = AdaptiveBlock;

  if (fromClientHints) {
    Block = media.width >= 1024 ? DesktopBlock : MobileBlock;
  }

  return <Block />;
};

Exported tokens

USER_AGENT_TOKEN

Object as a result of parsing user-agent string with @tinkoff/user-agent. Parsing happens only on server-side and parsed info is reused on client-side.

Env variables

TRAMVAI_USER_AGENT_CACHE_MAX

Size of User-Agent header parsing cache (in-memory LRU cache), default value is 200.

You can increase it for better cache hit rate and server performance.

Metrics

• user_agent_cache_gets counter - User-Agent header parsing count, with labels hit and miss