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'
};

buildEs5

Sets if the ES5 build should be generated or not. It defaults to false. Setting true will also create es5 builds for both dev and prod modes. Setting buildEs5 to prod will only build ES5 in prod mode. Basically if the app does not need to run on legacy browsers (IE11 and Edge 18 and below), it's safe to use the default respectively, buildEs5 set to false, which will also speed up production build times. In addition to creating es5 builds, apps may also be interested in enable runtime options to support legacy browsers. See config extras for more information.

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'] }
]

enableCache

default: true

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

enableCache: true

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();
}

The exported function can also be async.

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.

invisiblePrehydration

default: true

When true, invisiblePrehydration will use Ionic's opinion to 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. Please note that setting invisiblePrehydration to false will cause everything to be visible when your page is loaded, causing more prominent Flash of Unstyled Content (FOUC), however you can style your web component's fallback content to your preference.

invisiblePrehydration: true

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

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

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.

devServer

Please see the Dev-Server docs.

preamble

default: undefined

The preamble configuration is a string that represents a preamble in the main file of the build. Help to persist a banner or add relevant information about the resulting build.

preamble: 'Built with Stencil'

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.