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Stencil Config

In most cases, the stencil.config.ts file does not require any customization since Stencil comes with great default values out-of-the-box. In general, it's preferred to keep the config as minimal as possible. In fact, you could even delete the stencil.config.ts file entirely and an app would compile just fine. But at the same time, the compiler can be configured at the lowest levels using this config. Below are the many optional config properties.

Example stencil.config.ts:

import { Config } from '@stencil/core';

export const config: Config = {
  namespace: 'MyApp',
  srcDir: 'src'
};

buildDist​

default: true (prod), false (dev)

Sets whether or not Stencil will execute output targets and write output to dist/ when stencil build is called. Defaults to false when building for development and true when building for production. If set to true then Stencil will always build all output targets, regardless of whether the build is in dev or prod mode or using watch mode.

buildDist: true

buildEs5​

Sets if the ES5 build should be generated or not. It defaults to false. Setting buildEs5 to true will also create ES5 builds for both dev and prod modes. Setting buildEs5 to prod will only build ES5 in prod mode.

buildEs5: boolean | 'prod'

bundles​

By default, Stencil will statically analyze the application and generate a component graph of how all the components are interconnected. From the component graph it is able to best decide how components should be grouped depending on their usage with one another within the app. By doing so it's able to bundle components together in order to reduce network requests. However, bundles can be manually generated using the bundles config.

The bundles config is an array of objects that represent how components are grouped together in lazy-loaded bundles. This config is rarely needed as Stencil handles this automatically behind the scenes.

bundles: [
  { components: ['ion-button'] },
  { components: ['ion-card', 'ion-card-header'] }
]

cacheDir​

default: '.stencil'

The directory where sub-directories will be created for caching when enableCache is set true or if using Stencil's Screenshot Connector.

A Stencil config like the following:

stencil.config.ts
import { Config } from '@stencil/core';

export const config: Config = {
  ...,
  enableCache: true,
  cacheDir: '.cache',
  testing: {
    screenshotConnector: 'connector.js'
  }
}

Will result in the following file structure:

stencil-project-root
└── .cache
    ├── .build <-- Where build related file caching is written
    |
    └── screenshot-cache.json <-- Where screenshot caching is written

devServer​

Please see the Dev-Server docs.

enableCache​

default: true

Stencil will cache build results in order to speed up rebuilds. To disable this feature, set enableCache to false.

enableCache: true

extras​

Please see the Extras docs.

env​

default: {}

An object that can hold environment variables for your components to import and use. These variables can hold data objects depending on the environment you compile the components for. For example, let's say we want to provide an URL to our API based on a specific environment, we could provide it as such:

stencil.config.ts
import { Config } from '@stencil/core';

export const config: Config = {
  ...,
  env: {
    API_BASE_URL: process.env.API_BASE_URL
  }
}

Now when you build your components with this environment variable set, you can import it in your component as follows:

import { Component, h, Env, Host } from '@stencil/core';

@Component({
  tag: 'api-component',
})
export class APIComponent {
  async connectedCallback () {
    const res = await fetch(Env.API_BASE_URL)
    // ...
  }
}

globalScript​

The global script config option takes a file path as a string.

The global script runs once before your library/app loads, so you can do things like setting up a connection to an external service or configuring a library you are using.

The code to be executed should be placed within a default function that is exported by the global script. Ensure all of the code in the global script is wrapped in the function that is exported:

export default function() { // or export default async function()
  initServerConnection();
}
note

The exported function can also be async but be aware that this can have implications on the performance of your application as all rendering operations will be deferred until after the global script finishes.

globalStyle​

Stencil is traditionally used to compile many components into an app, and each component comes with its own compartmentalized styles. However, it's still common to have styles which should be "global" across all components and the website. A global CSS file is often useful to set CSS Variables.

Additionally, the globalStyle config can be used to precompile styles with Sass, PostCSS, etc.

Below is an example folder structure containing a webapp's global css file, named app.css.

src/
  components/
  global/
    app.css

The global style config takes a file path as a string. The output from this build will go to the buildDir. In this example it would be saved to www/build/app.css.

globalStyle: 'src/global/app.css'

Check out the styling docs of how to use global styles in your app.

hashedFileNameLength​

default: 8

When the hashFileNames config is set to true, and it is a production build, the hashedFileNameLength config is used to determine how many characters the file name's hash should be.

hashedFileNameLength: 8

hashFileNames​

default: true

During production builds, the content of each generated file is hashed to represent the content, and the hashed value is used as the filename. If the content isn't updated between builds, then it receives the same filename. When the content is updated, then the filename is different. By doing this, deployed apps can "forever-cache" the build directory and take full advantage of content delivery networks (CDNs) and heavily caching files for faster apps.

hashFileNames: true

hydratedFlag​

When using the lazy build Stencil has support for automatically applying a class or attribute to a component and all of its child components when they have finished hydrating. This can be used to prevent a flash of unstyled content (FOUC), a typically-undesired 'flicker' of unstyled HTML that might otherwise occur during component rendering while various components are asynchronously downloaded and rendered.

By default, Stencil will add the hydrated CSS class to elements to indicate hydration. The hydratedFlag config field allows this behavior to be customized, by changing the name of the applied CSS class, setting it to use an attribute to indicate hydration, or changing which type of CSS properties and values are assigned before and after hydrating. This config can also be used to turn off this behavior by setting it to null.

If a Stencil configuration does not supply a value for hydratedFlag then Stencil will automatically generate the following default configuration:

const defaultHydratedFlag: HydratedFlag = {
  hydratedValue: 'inherit',
  initialValue: 'hidden',
  name: 'hydrated',
  property: 'visibility',
  selector: 'class',
};

If hydratedFlag is explicitly set to null, Stencil will not set a default configuration and the behavior of marking hydration with a class or attribute will be disabled.

hydratedFlag: null | {
    name?: string,
    selector?: 'class' | 'attribute',
    property?: string,
    initialValue?: string,
    hydratedValue?: string
}

The supported options are as follows:

name​

default: 'hydrated'

The name which Stencil will use for the attribute or class that it sets on elements to indicate that they are hydrated.

name: string

selector​

default: 'class'

The way that Stencil will indicate that a component has been hydrated. When 'class', Stencil will set the name option on the element as a class, and when 'attribute', Stencil will similarly set the name option as an attribute.

selector: 'class' | 'attribute'

property​

default: 'visibility'

The CSS property used to show and hide components. This defaults to the CSS visibility property. Other possible CSS properties might include display with the initialValue setting as none, or opacity with the initialValue as 0. Defaults to visibility.

property: string

initialValue​

default: 'hidden'

This is the value which should be set for the property specified by property on all components before hydration.

initialValue: string

hydratedValue​

default: 'inherit'

This is the value which should be set for the property specified by property on all components once they've completed hydration.

hydratedValue: string

invisiblePrehydration​

default: true

When true, invisiblePrehydration will visually hide components before they are hydrated by adding an automatically injected style tag to the document's head. Setting invisiblePrehydration to false will not inject the style tag into the head, allowing you to style your web components pre-hydration.

note

Setting invisiblePrehydration to false will cause everything to be visible when your page is loaded, causing a more prominent Flash of Unstyled Content (FOUC). However, you can style your web component's fallback content to your preference.

invisiblePrehydration: true

minifyCss​

default: true in production

When true, the browser CSS file will be minified.

minifyJs​

default: true in production

When true, the browser JS files will be minified. Stencil uses Terser under-the-hood for file minification.

namespace​

default: App

The namespace config is a string representing a namespace for the app. For apps that are not meant to be a library of reusable components, the default of App is just fine. However, if the app is meant to be consumed as a third-party library, such as Ionic, a unique namespace is required.

namespace: "Ionic"

outputTargets​

Please see the Output Target docs.

plugins​

Please see the Plugin docs.

preamble​

default: undefined

Used to help to persist a banner or add relevant information about the resulting build, the preamble configuration field is a string that will be converted into a pinned comment and placed at the top of all emitted JavaScript files, with the exception of any emitted polyfills. Escaped newlines may be placed in the provided value for this field and will be honored by Stencil.

Example:

preamble: 'Built with Stencil\nCopyright (c) SomeCompanyInc.'

Will generate the following comment:

/*!
 * Built with Stencil
 * Copyright (c) SomeCompanyInc.
 */

sourceMap​

default: true

When omitted or set to true, sourcemaps will be generated for a project. When set to false, sourcemaps will not be generated.

sourceMap: true | false

Sourcemaps create a translation between Stencil components that are written in TypeScript/JSX and the resulting JavaScript that is output by Stencil. Enabling source maps in your project allows for an improved debugging experience for Stencil components. For example, they allow external tools (such as an Integrated Development Environment) to add breakpoints directly in the original source code, which allows you to 'step through' your code line-by-line, to inspect the values held in variables, to observe logic flow, and more.

Please note: Stencil will always attempt to minify a component's source code as much as possible during compilation. When sourceMap is enabled, it is possible that a slightly different minified result will be produced by Stencil when compared to the minified result produced when sourceMap is not enabled.

Developers are responsible for determining whether or not they choose to serve sourcemaps in each environment their components are served and implementing their decision accordingly.

srcDir​

default: src

The srcDir config specifies the directory which should contain the source typescript files for each component. The standard for Stencil apps is to use src, which is the default.

srcDir: 'src'

taskQueue​

default: async

Sets the task queue used by stencil's runtime. The task queue schedules DOM read and writes across the frames to efficiently render and reduce layout thrashing. By default, the async is used. It's recommended to also try each setting to decide which works best for your use-case. In all cases, if your app has many CPU intensive tasks causing the main thread to periodically lock-up, it's always recommended to try Web Workers for those tasks.

  • congestionAsync: DOM reads and writes are scheduled in the next frame to prevent layout thrashing. When the app is heavily tasked and the queue becomes congested it will then split the work across multiple frames to prevent blocking the main thread. However, it can also introduce unnecessary reflows in some cases, especially during startup. congestionAsync is ideal for apps running animations while also simultaneously executing intensive tasks which may lock-up the main thread.

  • async: DOM read and writes are scheduled in the next frame to prevent layout thrashing. During intensive CPU tasks it will not reschedule rendering to happen in the next frame. async is ideal for most apps, and if the app has many intensive tasks causing the main thread to lock-up, it's recommended to try Web Workers rather than the congestion async queue.

  • immediate: Makes writeTask() and readTask() callbacks to be executed synchronously. Tasks are not scheduled to run in the next frame, but do note there is at least one microtask. The immediate setting is ideal for apps that do not provide long-running and smooth animations. Like the async setting, if the app has intensive tasks causing the main thread to lock-up, it's recommended to try Web Workers.

taskQueue: 'async'

testing​

Please see the testing config docs.

transformAliasedImportPaths​

default: true

This sets whether or not Stencil should transform path aliases set in a project's tsconfig.json from the assigned module aliases to resolved relative paths. This will not transform external imports (like @stencil/core) or relative imports (like '../utils').

This option applies globally and will affect all code processed by Stencil, including .d.ts files and spec tests.

An example of path transformation could look something like the following.

First, a set of paths aliases in tsconfig.json:

tsconfig.json
{
  "compilerOptions": {
    "paths": {
      "@utils": [
        "../path/to/utils"
      ]
    }
  }
}

Then with the following input:

src/my-module.ts
import { utilFunc, UtilInterface } from '@utils'

export function util(arg: UtilInterface) {
    utilFunc(arg)
}

Stencil will produce the following output:

dist/my-module.js
import { utilFunc } from '../path/to/utils';
export function util(arg) {
    utilFunc(arg);
}
dist/my-module.d.ts
import { UtilInterface } from '../path/to/utils';
export declare function util(arg: UtilInterface): void;

validatePrimaryPackageOutputTarget​

default: false

When true, validation for common package.json fields will occur based on setting an output target's isPrimaryPackageOutputTarget flag. For more information on package validation, please see the output target docs.

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