next/image gives false warning for sizes: 100vw

[cibulka]

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/unruffled-haze-nnxhyk?file=%2Fapp%2Fpage.tsx%3A4%2C28

To Reproduce

1. Start the application in dev mode (npm run dev)
2. Include the Next/Image like this:

<Image 
      alt="Photo" 
      src="https://images.unsplash.com/photo-1682686581484-a220483e6291?q=80&w=2970&auto=format&fit=crop&ixlib=rb-4.0.3&ixid=M3wxMjA3fDF8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D" 
      fill 
      sizes="100vw" 
      style={{ objectFit: 'cover' }}
/>

3. Receive the following warning: Image with src "https://images.unsplash.com/photo-1682686581484-a220483e6291?q=80&w=2970&auto=format&fit=crop&ixlib=rb-4.0.3&ixid=M3wxMjA3fDF8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D" has "fill" but is missing "sizes" prop. Please add it to improve page performance.

Current vs. Expected behavior

Current behaviour

Next/Image with sizes="100vw" and fill! gives the following warning: Image with src "https://placehold.co/600x400" has "fill" but is missing "sizes" prop. Please add it to improve page performance.`

Epx

Having the attribute sizes marked as 100vw is a valid use-case for full-page images (such as backgrounds). According to developer.mozilla.org there is no other way how to express 100% of viewport for this attribute than 100vw.

Note: The source size value must not be specified as a percentage of the container size; that is, lengths such as 50% or 100% are not allowed, as there would be uncertainty as to what the specified value is a percentage of.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:43 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T6000
Binaries:
  Node: 20.9.0
  npm: 10.1.0
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 14.0.3
  eslint-config-next: 14.0.3
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

App Router, Image optimization (next/image, next/legacy/image)

Additional context

In a Next.js dicusssion there are some solutions proposed that remove the error, such as using values as 100% or auto. They remove the warning, but none of them is a valid usage of sizes attribute.

_NEXT-2441

[github-actions]

We cannot recreate the issue with the provided information. Please add a reproduction in order for us to be able to investigate.

Why was this issue marked with the please add a complete reproduction label?

To be able to investigate, we need access to a reproduction to identify what triggered the issue. We prefer a link to a public GitHub repository (template for App Router, template for Pages Router), but you can also use these templates: CodeSandbox: App Router or CodeSandbox: Pages Router.

To make sure the issue is resolved as quickly as possible, please make sure that the reproduction is as minimal as possible. This means that you should remove unnecessary code, files, and dependencies that do not contribute to the issue. Ensure your reproduction does not depend on secrets, 3rd party registries, private dependencies, or any other data that cannot be made public. Avoid a reproduction including a whole monorepo (unless relevant to the issue). The easier it is to reproduce the issue, the quicker we can help.

Please test your reproduction against the latest version of Next.js (next@canary) to make sure your issue has not already been fixed.

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I added a link, why was it still marked?

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What happens if I don't provide a sufficient minimal reproduction?

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I did not open this issue, but it is relevant to me, what can I do to help?

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Useful Resources

• How to Contribute to Open Source (Next.js)
• How to create a Minimal, Complete, and Verifiable example
• Reporting a Next.js bug
• Next.js Triaging issues

[styfle]

I visited the link and can't reproduce your issue.

[styfle]

Having the attribute sizes marked as 100vw is a valid use-case for full-page images (such as backgrounds).

Please note that the warning is only printed when the image is rendered less than 60vw, meaning 100vw is too large and should be adjusted.

next.js/packages/next/src/client/image-component.tsx

Lines 118 to 129 in abe8b1e

	if (
	!unoptimized &&
	(!img.getAttribute('sizes') || img.getAttribute('sizes') === '100vw')
	) {
	let widthViewportRatio =
	img.getBoundingClientRect().width / window.innerWidth
	if (widthViewportRatio < 0.6) {
	warnOnce(
	`Image with src "${origSrc}" has "fill" but is missing "sizes" prop. Please add it to improve page performance. Read more: https://nextjs.org/docs/api-reference/next/image#sizes`
	)
	}
	}

[cibulka]

Having the attribute sizes marked as 100vw is a valid use-case for full-page images (such as backgrounds).

Please note that the warning is only printed when the image is rendered less than 60vw, meaning 100vw is too large and should be adjusted.

next.js/packages/next/src/client/image-component.tsx

Lines 118 to 129 in abe8b1e

	if (
	!unoptimized &&
	(!img.getAttribute('sizes') || img.getAttribute('sizes') === '100vw')
	) {
	let widthViewportRatio =
	img.getBoundingClientRect().width / window.innerWidth
	if (widthViewportRatio < 0.6) {
	warnOnce(
	`Image with src "${origSrc}" has "fill" but is missing "sizes" prop. Please add it to improve page performance. Read more: https://nextjs.org/docs/api-reference/next/image#sizes`
	)
	}
	}

I see! In that case I believe the warning is misleading. The image is not missing the sizes prop, it just does not have appropriate value. Maybe it would make sense to adjust the error message?

[styfle]

Good point! Fixing in PR #61949

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.