feat: support custom image lazy loading (#3346)

Co-authored-by: Divyansh Singh <[email protected]>

Author: zonemeen

Diff for: __tests__/e2e/.vitepress/config.ts

@@ -86,6 +86,11 @@ const sidebar: DefaultTheme.Config['sidebar'] = {
 export default defineConfig({
   title: 'Example',
   description: 'An example app using VitePress.',
+  markdown: {
+    image: {
+      lazyLoading: true
+    }
+  },
   themeConfig: {
     sidebar,
     search: {

Diff for: __tests__/e2e/markdown-extensions/index.md

@@ -196,3 +196,7 @@ export default config
 ## Markdown File Inclusion with Range without End
 
 <!--@include: ./foo.md{6,}-->
+
+## Image Lazy Loading
+
+![vitepress logo](/vitepress.png)

Diff for: __tests__/e2e/markdown-extensions/markdown-extensions.test.ts

@@ -65,7 +65,7 @@ describe('Table of Contents', () => {
   test('render toc', async () => {
     const items = page.locator('#table-of-contents + nav ul li')
     const count = await items.count()
-    expect(count).toBe(35)
+    expect(count).toBe(36)
   })
 })
 
@@ -280,3 +280,10 @@ describe('Markdown File Inclusion', () => {
     expect(await p.textContent()).not.toContain('title')
   })
 })
+
+describe('Image Lazy Loading', () => {
+  test('render loading="lazy" in the <img> tag', async () => {
+    const img = page.locator('#image-lazy-loading + p img')
+    expect(await img.getAttribute('loading')).toBe('lazy')
+  })
+})

Diff for: docs/guide/markdown.md

@@ -847,6 +847,21 @@ $$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$
 | $\nabla \times \vec{\mathbf{E}}\, +\, \frac1c\, \frac{\partial\vec{\mathbf{B}}}{\partial t}  = \vec{\mathbf{0}}$                                                          | curl of $\vec{\mathbf{E}}$ is proportional to the rate of change of $\vec{\mathbf{B}}$ |
 | $\nabla \times \vec{\mathbf{B}} -\, \frac1c\, \frac{\partial\vec{\mathbf{E}}}{\partial t} = \frac{4\pi}{c}\vec{\mathbf{j}}    \nabla \cdot \vec{\mathbf{E}} = 4 \pi \rho$ | _wha?_                                                                                 |
 
+## Image Lazy Loading
+
+You can enable lazy loading for each image added via markdown by setting `lazyLoading` to `true` in your config file:
+
+```js
+export default {
+  markdown: {
+    image: {
+      // image lazy loading is disabled by default
+      lazyLoading: true
+    }
+  }
+}
+```
+
 ## Advanced Configuration
 
 VitePress uses [markdown-it](https://github.com/markdown-it/markdown-it) as the Markdown renderer. A lot of the extensions above are implemented via custom plugins. You can further customize the `markdown-it` instance using the `markdown` option in `.vitepress/config.js`:

Diff for: src/node/markdown/markdown.ts

@@ -23,7 +23,7 @@ import type { Logger } from 'vite'
 import { containerPlugin, type ContainerOptions } from './plugins/containers'
 import { highlight } from './plugins/highlight'
 import { highlightLinePlugin } from './plugins/highlightLines'
-import { imagePlugin } from './plugins/image'
+import { imagePlugin, type Options as ImageOptions } from './plugins/image'
 import { lineNumberPlugin } from './plugins/lineNumbers'
 import { linkPlugin } from './plugins/link'
 import { preWrapperPlugin } from './plugins/preWrapper'
@@ -166,6 +166,7 @@ export interface MarkdownOptions extends MarkdownIt.Options {
    * @see https://vitepress.dev/guide/markdown#math-equations
    */
   math?: boolean | any
+  image?: ImageOptions
 }
 
 export type MarkdownRenderer = MarkdownIt
@@ -198,7 +199,7 @@ export const createMarkdownRenderer = async (
     .use(preWrapperPlugin, { hasSingleTheme })
     .use(snippetPlugin, srcDir)
     .use(containerPlugin, { hasSingleTheme }, options.container)
-    .use(imagePlugin)
+    .use(imagePlugin, options.image)
     .use(
       linkPlugin,
       { target: '_blank', rel: 'noreferrer', ...options.externalLinks },

Diff for: src/node/markdown/plugins/image.ts

@@ -3,7 +3,15 @@
 import type MarkdownIt from 'markdown-it'
 import { EXTERNAL_URL_RE } from '../../shared'
 
-export const imagePlugin = (md: MarkdownIt) => {
+export interface Options {
+  /**
+   * Support native lazy loading for the `<img>` tag.
+   * @default false
+   */
+  lazyLoading?: boolean
+}
+
+export const imagePlugin = (md: MarkdownIt, { lazyLoading }: Options = {}) => {
   const imageRule = md.renderer.rules.image!
   md.renderer.rules.image = (tokens, idx, options, env, self) => {
     const token = tokens[idx]
@@ -12,6 +20,9 @@ export const imagePlugin = (md: MarkdownIt) => {
       if (!/^\.?\//.test(url)) url = './' + url
       token.attrSet('src', decodeURIComponent(url))
     }
+    if (lazyLoading) {
+      token.attrSet('loading', 'lazy')
+    }
     return imageRule(tokens, idx, options, env, self)
   }
 }