ngx-preload-fonts

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Angular post-build tool to inject font preload links into index file of your app. Works for any font handled by the Angular build system - fingerprinted or not.

What problem does this library solve?

One of the Google Lighthouse suggestions for increasing the performance of your app will likely be to preload your fonts by adding a simple tag for each of your fonts in your index.html:

<link rel="preload" as="font" href="helvetica.831fa2178a6e738.woff2" type="font/woff2" crossorigin="anonymous">

Seems like an easy task, until you realize, that Angular fingerprints your fonts with a unique hash at build time. This hash will be included in the font name and it will change every time you change the content of the file (e.g. when updating the character set). Angular will automatically update references in stylesheets to use the generated fingerprinted font names, but it will not update the links in your index.html.

This library provides a command line tool that will pick up all fonts from the root of your build output directory and inserts preload links for them into the index file of the build. The tool also gives control to omit the preload links for a subset of the fonts.

Limitations of this tool

This tool is laid out to automatically find fonts, which are handled by Angular build system. Specifically, this will include fonts which are referenced by relative paths in your stylesheets or stylesheets you imported from libraries.

Angular will copy these fonts to the root of the build output directory, next to the index file. Therefore ngx-preload-fonts will crawl this root directory only.

If you use absolutely referenced fonts somewhere, they are not managed by Angular and you will most likely have some angular.json assets configuration that copies these fonts somewhere into the build output directory. ngx-preload-fonts will only find them, unless they are located in the root next to the index file. It does not crawl the subdirectories. If you would like to see this feature, give me a feature request. But I assume that when you use absolute paths, the URLs are being static anyway (as Angular would not fingerprint them) and the preload links could be declared manually.

Background on Angular CSS resource handling and fingerprinting

You can skip this section, if you just want to get your job done :)

The official Angular docs contain only one sentence concerning the fingerprinting and the CSS resource handling during the Angular build:

"Resources in CSS, such as images and fonts, are automatically written and fingerprinted at the root of the output folder."

This sentence is not very precise, as it does not conceal the auxiliary conditions, which must be fulfilled for these resources to be picked up and handled by Angular. This causes confusion that is reflected in multiple GitHub issue discussions (as seen here, here and here) which provide valuable insight that is missing in the docs. Here is what I've learned from these discussions and from playing with different angular.json settings.

Turning Fingerprinting on/off

Fingerprinting can be turned on/off easily via the property outputHashing: non|all|bundles|media in angular.json. The disadvantage is the granularity of the property. When choosing bundles, only your JS bundles are being fingerprinted. Fonts, images and other media is not. The property does not provide a way to turn off fingerprinting exclusively for fonts. Since fingerprinting serves a valuable purpose for carefree caching, you should ponder thoroughly if turning it off might not kick your ass in the long run.

By the way, the property outputHashing used to work for ng serve as well, but since Angular 11 it only enabled for production builds and a warning shows up if the option is active for ng serve configuration. This seems to be fixed soon, so that hashing can be activated again locally. Until then, to debug the effects of the outputHashing property and the differences of relative and absolute paths in stylesheets, you need to build your project and inspect the generated build artefact.

Consequences of relative resource paths

When fingerprinting is turned on, Angular build system automatically fingerprints resources (fonts, images, …) which are referenced via a relative path in a stylesheet (stylesheet of the project or a stylesheet imported from a library). More precisely: when Angular sees a relative path for a resource in a stylesheet, Angular will pick up and handle this resource during the build. Angular will automatically take care of copying that resource into the build output directory. It does not matter where the resource is located (project, workspace library or node_modules package). Some developers prefer to locate fonts and media resources near the components which use them, so they might be spread across multiple directories of the application. If they are referenced relatively, Angular will pick them up, no matter where they are.

Besides the fingerprinting and the auto-copying, Angular build system is able to process the resource for optimization purposes. In a Github Issue discussion dating back to 2018, a supposedly Angular employee indicated, that at that point, there was no processing other than fingerprinting being performed. But this might change in the future or might have already changed by now.

Consequences of absolute resource paths

Absolute paths for resources in stylesheets will have the consequence that Angular ignores these resources during the build completely. Therefore they will not be fingerprinted, even when fingerprinting is activated via outputHashing property and they will also not being copied automatically into the build output directory. You need to manually configure that the resources are being copied into the build output directory, which can be easily accomplished in angular.json via the assets property in the build architect section. Here you can define paths or glob patterns of resources to be copied into the build artifact:

"assets": [
    "projects/my-app/src/favicon.ico",
    "projects/my-app/src/assets",
    "projects/my-app/src/manifests",
    {
    "glob": "**/*",
    "input": "./node_modules/@my-company/my-font-lib/dist/assets",
    "output": "./../assets"
    }
],

But this manual configuration can become quite complex, when your fonts are cluttered across different directories. It enforces to keep the fonts together in a small number of directories, which means locating them near the components or modules which use them, is not a good choice when using absolute resource paths.

Custom base href

From what I've experienced, Angular manages relative paths and a custom base href without problems. However, the custom base href which is defined at runtime, makes it harder to define absolute paths. Also I ran into a problem when I tried to access resources with the local dev server that were located outside of the base href environment. For example, in a typical Angular i18n application your base href will be the locale. Angular will generate one build per locale. It might be tempting to share some resources like fonts or images across these builds and have them located in a shared assets directory next to the build directories for the locales:

/dist
|_/my-app
  |_/de
    |_/assets
    |_main.js
    |_index.html
  |_/en
    |_/assets
    |_main.js
    |_index.html
  |_/assets
    |_helvetica.woff2

The absolute path to reference the font from the shared assets directory did not include the base href and in local development mode with ng serve, Angular was not able to serve this file. It probalby would have worked in production where NGINX is used to serve the files, but it is not really an option to use a solution which does not work in development mode.

Summary and Best Practice

Use relative paths in your stylesheets to reference fonts and media resources, like these:

@font-face {
  font-family: 'Raleway';
  src: url('~/assets/fonts/raleway/raleway-v19-latin-ext_latin-200.woff2') format('woff2'),
       url('~/assets/fonts/raleway/raleway-v19-latin-ext_latin-200.woff') format('woff'),
       url('~/assets/fonts/raleway/raleway-v19-latin-ext_latin-200.ttf') format('truetype'),
}

It will allow Angular build system to pick up and manage these resources during build, which gives you these benefits:

• Automatic fingerprinting for carefree caching (if enabled via outputHashing property in angular.json)
• Resources will automatically end up in the build output directory without manual configuration and regardless whether they are located in a random application subdirectory, an imported workspace library or an imported library within node_modules
• Automatic file optimization (not sure what Angular has implemented at this point)
• Good integration with custom base href

Use ngx-preload-fonts to tackle the only problem with fingerprinted fonts: preloading them :)

How to use

Install the tool as a dev dependency:

npm install --save-dev @bfoese/ngx-preload-fonts

Add a script in package.json that calls the tool in the postbuild step - option #1 should work, if not, you can use option #2:

# Option 1: via registered command
"postbuild": "preloadfonts --dist dist/my-app -f index.html -i arial,helvetica -e comic-sans",

# Option 2: via path
"preload:fonts": "ts-node -r tsconfig-paths/register ./node_modules/@bfoese/ngx-preload-fonts",
"postbuild": "npm run preload:fonts -- --dist dist/my-app -f index.html -i arial,helvetica -e comic-sans"

CLI Options	Shortcut	Optional	Purpose
--dist	-d	no	Path to your build output directory. Should be identical with outputPath property from angular.json
--file	-f	yes	Name of your index file if it is something else than index.html. Should be identical with index property from angular.json
--include	-i	yes	Optional list of font names (without file type and fingerprint). If provided, all other fonts will be ignored for the link creation.
--exclude	-e	yes	Optional list of font names (without file type and fingerprint). If provided, these fonts will be ignored for the link creation.
--help	-h	yes	Print out all available options for the CLI tool

The only other thing you need to do, is to insert a static injection marker into your index file, which will tell the tool where you want the preload links to be generated. This is the marker, the tool is looking for:

<!-- inject:preload-fonts --><!-- endinject -->

The marker should be included in the <head> section and must be located somewhere after the <base href> tag:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>My fancy app</title>
    <base href="/"/>

    <!-- inject:preload-fonts --><!-- endinject -->
  </head>
  <body></body>
</html>

To test the setup, you can run:

npm run build

When the postbuild step is finished, you should find a summary in the console log with the number of preload links being created. Afterwards you can check the content of the index file(s) in your build output directory.

Multi-builds

The tool can also be used in multi-build setups where Angular builds multiple versions of your app at once. For example when using Angular i18n, you can have one build for each locale.

In angular.json you defined:

"architect": {
  "build": {
    "options": {
      "outputPath": "dist/my-app",
      "index": "projects/my-app/src/index.html",
    }
  }
}

Then the build output directory structure will probably look like this:

/dist
|_/my-app
  |_/de
    |_arial.a9879asf89s7d.woff2
    |_index.html
  |_/en
    |_arial.a9879asf89s7d.woff2
    |_index.html

If your goal is to generate prebuild links in both index.html files, you should run the postbuild tool with the build output root dir path:

preloadfonts --dist dist/my-app -f index.html

If your goal is to generate the prebuild links only for the app version with locale en, then you should run the postbuild tool with the path of that app version:

preloadfonts --dist dist/my-app/en -f index.html