Assets

Stencil components may need one or more static files as a part of their design. These types of files are referred to as 'assets', and include images, fonts, etc.

In this guide, we describe different strategies for resolving assets on the filesystem.

note

CSS files are handled differently than assets; for more on using CSS, please see the styling documentation.

Asset Base Path

The asset base path is the directory that Stencil will use to resolve assets. When a component uses an asset, the asset's location is resolved relative to the asset base path.

The asset base path is automatically set for the following output targets:

• dist
• hydrate
• www

For all other output targets, assets must be moved and the asset base path must be manually set.

For each instance of the Stencil runtime that is loaded, there is a single asset base path. Oftentimes, this means there is only one asset base path per application using Stencil.

Resolution Overview

The process of resolving an asset involves asking Stencil to build a path to the asset on the filesystem.

When an asset's path is built, the resolution is always done in a project's compiled output, not the directory containing the original source code.

The example below uses the output of the www output target to demonstrate how assets are resolved. Although the example uses the output of www builds, the general principle of how an asset is found holds for all output targets.

When using the www output target, a build/ directory is automatically created and set as the asset base path. An example build/ directory and the assets it contains can be found below.

www/
├── build/
│   ├── assets/
│   │   ├── logo.png
│   │   └── scenery/
│   │       ├── beach.png
│   │       └── sunset.png
│   └── other-assets/
│       └── font.tiff
└── ...

To resolve the path to an asset, Stencil's getAssetPath() API may be used. When using getAssetPath, the assets in the directory structure above are resolved relative to build/.

The code sample below demonstrates the return value of getAssetPath for different path arguments. The return value is a path that Stencil has built to retrieve the asset on the filesystem.

// with an asset base path of "/build/":

// '/build/assets/logo.png'
getAssetPath('assets/logo.png');
// '/build/assets/scenery/beach.png'
getAssetPath('assets/scenery/beach.png');
// '/build/other-assets/font.tiff'
getAssetPath('other-assets/font.tiff'); 

Making Assets Available

In order to be able to find assets at runtime, they need to be found on the filesystem from the output of a Stencil build. In other words, we need to ensure they exist in the distribution directory. This section describes how to make assets available under the asset base path.

assetsDirs

The @Component decorator can be configured with the assetsDirs option. assetsDirs takes an array of strings, where each entry is a relative path from the component to a directory containing the assets the component requires.

When using the dist or www output targets, setting assetsDirs instructs Stencil to copy that folder into the distribution folder. When using other output targets, Stencil will not copy assets into the distribution folder.

Below is an example project's directory structure containing an example component and an assets directory.

src/
└── components/
    ├── assets/
    │   ├── beach.jpg
    │   └── sunset.jpg
    └── my-component.tsx

Below, the my-component component will correctly load the assets based on it's image prop.

// file: my-component.tsx
// 1. getAssetPath is imported from '@stencil/core'
import { Component, Prop, getAssetPath, h } from '@stencil/core';

@Component({
  tag: 'my-component',
  // 2. assetsDirs lists the 'assets' directory as a relative
  //    (sibling) directory
  assetsDirs: ['assets']
})
export class MyComponent {

  @Prop() image = "sunset.jpg";

  render() {
    // 3. the asset path is retrieved relative to the asset 
    //    base path to use in the <img> tag
    const imageSrc = getAssetPath(`./assets/${this.image}`);
    return <img src={imageSrc} />
  }
}

In the example above, the following allows my-component to display the provided asset:

1. getAssetPath() is imported from @stencil/core
2. The my-component's component decorator has the assetsDirs property, and lists the sibling directory, assets. This will copy assets over to the distribution directory.
3. getAssetPath is used to retrieve the path to the image to be used in the <img> tag

Manually Moving Assets

For the dist-custom-elements output target, options like assetsDirs do not copy assets to the distribution directory.

It's recommended that a bundler (such as rollup) or a Stencil copy task is used to ensure the static assets are copied to the distribution directory.

Stencil Copy Task

Stencil copy tasks can be used to define files and folders to be copied over to the distribution directory.

The example below shows how a copy task can be used to find all '.jpg' and '.png' files under a project's src directory and copy them to dist/components/assets at build time.

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

export const config: Config = {
  namespace: 'your-component-library',
  outputTargets: [
    {
      type: 'dist-custom-elements',
      copy: [
        {
          src: '**/*.{jpg,png}',
          dest: 'dist/components/assets',
          warn: true,
        }
      ]
    },
  ], 
  // ...
};

Rollup Configuration

Rollup Plugins's can be used to define files and folders to be copied over to the distribution directory.

The example below shows how a the rollup-plugin-copy NPM module can be used to find all '.jpg' and '.png' files under a project's src directory and copy them to dist/components/assets at build time.

import { Config } from '@stencil/core';
import copy from 'rollup-plugin-copy';

export const config: Config = {
    namespace: 'copy',
    outputTargets: [
      {
        type: 'dist-custom-elements',
      },
    ],
    rollupPlugins: {
      after: [
        copy({
          targets: [
            {
              src: 'src/**/*.{jpg,png}',
              dest: 'dist/components/assets',
            },
          ],
        }),
      ]
    }
};

API Reference

getAssetPath

getAssetPath() is an API provided by Stencil to build the path to an asset, relative to the asset base path.

/** 
 * Builds a URL to an asset. This is achieved by combining the 
 * provided `path` argument with the base asset path.
 * @param path the path of the asset to build a URL to
 * @returns the built URL
 */
declare function getAssetPath(path: string): string;

The code sample below demonstrates the return value of getAssetPath for different path arguments, when an asset base path of /build/ has been set.

// with an asset base path of "/build/":
// "/build/"
getAssetPath('');
// "/build/my-image.png"
getAssetPath('my-image.png');
// "/build/assets/my-image.png"
getAssetPath('assets/my-image.png');
// "/build/assets/my-image.png"
getAssetPath('./assets/my-image.png');
// "/assets/my-image.png"
getAssetPath('../assets/my-image.png');
// "/assets/my-image.png"
getAssetPath('/assets/my-image.png');

setAssetPath

setAssetPath is an API provided by Stencil to manually set the asset base path where assets can be found.

/**
 * Set the base asset path for resolving components
 * @param path the base asset path
 * @returns the new base asset path
 */
export declare function setAssetPath(path: string): string;

Calling this API will set the asset base path for all Stencil components attached to a Stencil runtime. As a result, calling setAssetPath should not be done from within a component in order to prevent unwanted side effects when using a component.

If the file calling setAssetPath is a module, it's recommended to use import.meta.url.

Alternatively, one may use document.currentScript.src when working in the browser and not using modules or environment variables (e.g. document.env.ASSET_PATH) to set the asset base path. This configuration depends on how your script is bundled, (or lack of bundling), and where your assets can be loaded from.