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fix(render): strip React 19 auto-injected image preload links #3109
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80 changes: 80 additions & 0 deletions
80
packages/render/src/shared/utils/strip-preload-links.spec.ts
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,80 @@ | ||
| import { stripAutoInjectedImagePreloads } from './strip-preload-links'; | ||
|
|
||
| describe('stripAutoInjectedImagePreloads', () => { | ||
| it('removes image preload links that match an <img> src', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="img/test.png"/><h1>Hello</h1><img src="img/test.png"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<h1>Hello</h1><img src="img/test.png"/>', | ||
| ); | ||
| }); | ||
|
|
||
| it('removes multiple matching image preload links', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="a.png"/><link rel="preload" as="image" href="b.png"/><img src="a.png"/><img src="b.png"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<img src="a.png"/><img src="b.png"/>', | ||
| ); | ||
| }); | ||
|
|
||
| it('preserves image preload links that do NOT match any <img> src', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="bg-pattern.png"/><img src="logo.png"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe(html); | ||
| }); | ||
|
|
||
| it('preserves non-image preload links (fonts, scripts, etc.)', () => { | ||
| const html = | ||
| '<link rel="preload" as="font" href="font.woff2" crossorigin/><link rel="preload" as="script" href="app.js"/><img src="logo.png"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe(html); | ||
| }); | ||
|
|
||
| it('returns unchanged html when no images exist', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="test.png"/><h1>Hello</h1>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe(html); | ||
| }); | ||
|
|
||
| it('returns unchanged html when no preload links exist', () => { | ||
| const html = '<h1>Hello</h1><img src="test.png"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe(html); | ||
| }); | ||
|
|
||
| it('handles self-closing tags with a space before the slash', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="test.png" /><img src="test.png" />'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<img src="test.png" />', | ||
| ); | ||
| }); | ||
|
|
||
| it('removes srcset-based image preloads that match an <img> srcset', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" imagesrcset="img-1x.png 1x, img-2x.png 2x"/><img srcset="img-1x.png 1x, img-2x.png 2x"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<img srcset="img-1x.png 1x, img-2x.png 2x"/>', | ||
| ); | ||
| }); | ||
|
|
||
| it('removes srcset-based preloads with imagesizes', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" imagesrcset="sm.png 480w, lg.png 800w" imagesizes="(max-width: 600px) 480px, 800px"/><img srcset="sm.png 480w, lg.png 800w" sizes="(max-width: 600px) 480px, 800px"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<img srcset="sm.png 480w, lg.png 800w" sizes="(max-width: 600px) 480px, 800px"/>', | ||
| ); | ||
| }); | ||
|
|
||
| it('preserves srcset-based preloads that do NOT match any <img> srcset', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" imagesrcset="other.png 1x"/><img srcset="logo.png 1x"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe(html); | ||
| }); | ||
|
|
||
| it('handles mixed src and srcset images', () => { | ||
| const html = | ||
| '<link rel="preload" as="image" href="a.png"/><link rel="preload" as="image" imagesrcset="b-1x.png 1x, b-2x.png 2x"/><img src="a.png"/><img srcset="b-1x.png 1x, b-2x.png 2x"/>'; | ||
| expect(stripAutoInjectedImagePreloads(html)).toBe( | ||
| '<img src="a.png"/><img srcset="b-1x.png 1x, b-2x.png 2x"/>', | ||
| ); | ||
| }); | ||
| }); |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,70 @@ | ||
| /** | ||
| * Strips auto-injected image preload links from rendered HTML. | ||
| * | ||
| * React 19's server renderer automatically injects preload links for every | ||
| * `<img>` it encounters during SSR: | ||
| * | ||
| * - `<img src="X">` → `<link rel="preload" as="image" href="X" />` | ||
| * - `<img srcset="X 1x">` → `<link rel="preload" as="image" imagesrcset="X 1x" />` | ||
| * | ||
| * These are useful for web pages but unnecessary in email HTML, where they add | ||
| * noise and are ignored by email clients. | ||
| * | ||
| * To avoid removing preload links that the template author added intentionally | ||
| * (e.g., font or script preloads, or image preloads for resources not rendered | ||
| * via `<img>`), we only strip image preloads whose href/imagesrcset matches an | ||
| * `<img src>`/`<img srcset>` found in the document. | ||
| * | ||
| * Known limitation: if a template author explicitly adds | ||
| * `<link rel="preload" as="image" href="X">` inside `<Head>` for the exact | ||
| * same `X` that also appears as an `<img src="X">`, it will be stripped. | ||
| * This is acceptable because image preloads are not supported by email clients | ||
| * and have no effect in rendered emails. | ||
| */ | ||
| export const stripAutoInjectedImagePreloads = (html: string): string => { | ||
| // Collect every <img src="..."> and <img srcset="..."> in the document. | ||
| const imgSrcs = new Set<string>(); | ||
| const imgSrcSets = new Set<string>(); | ||
|
|
||
| const imgPattern = /<img\b[^>]*?\/?>/gi; | ||
| let imgMatch: RegExpExecArray | null; | ||
| while ((imgMatch = imgPattern.exec(html)) !== null) { | ||
| const tag = imgMatch[0]; | ||
|
|
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| const srcMatch = /\bsrc=["']([^"']+)["']/i.exec(tag); | ||
|
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|
||
| if (srcMatch) { | ||
| imgSrcs.add(srcMatch[1]!); | ||
| } | ||
|
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| const srcSetMatch = /\bsrcset=["']([^"']+)["']/i.exec(tag); | ||
| if (srcSetMatch) { | ||
| imgSrcSets.add(srcSetMatch[1]!); | ||
| } | ||
| } | ||
|
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| if (imgSrcs.size === 0 && imgSrcSets.size === 0) return html; | ||
|
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| // Remove only <link rel="preload" as="image"> whose href or imagesrcset | ||
| // matches a rendered <img> — these are the ones React 19 auto-injects. | ||
| return html.replace( | ||
| /<link[^>]*?\s+rel="preload"[^>]*?\/?>/gi, | ||
| (tag) => { | ||
| const isImage = /\bas=["']image["']/i.test(tag); | ||
| if (!isImage) return tag; | ||
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| // React 19 emits href-based preloads for <img src>. | ||
| const hrefMatch = /\bhref=["']([^"']+)["']/i.exec(tag); | ||
| if (hrefMatch && imgSrcs.has(hrefMatch[1]!)) { | ||
| return ''; | ||
| } | ||
|
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| // React 19 emits imagesrcset-based preloads for <img srcset>. | ||
| const imgSrcSetMatch = /\bimagesrcset=["']([^"']+)["']/i.exec(tag); | ||
| if (imgSrcSetMatch && imgSrcSets.has(imgSrcSetMatch[1]!)) { | ||
| return ''; | ||
| } | ||
|
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||
| return tag; | ||
| }, | ||
| ); | ||
| }; | ||
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I really don't like this much regex, we should find some react-dom native way of not having the links anymore.
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Hey @gabrielmfern , thanks for the feedback! I totally understand the concern about the regex approach.
I did some research into React's internals to find a native solution.
Unfortunately, there's i havent found a global option in renderToReadableStream or renderToPipeableStream to disable auto-injected preloads.
However, looking at the source code in ReactFizzConfigDOM.js (pushImg function), the preload is suppressed when any of these conditions are met on the![]()
:
One idea: we could add fetchPriority="low" to the![]()
component email clients ignore this attribute, so it wouldn't affect rendering. The trade-off is that it adds a fetchPriority="low" attribute to every ![]()
in the output HTML.
What do you think?