next export with exportPathMap in next.config.js creates a duplicate 404.html.html

[mdubourg001]

Verify canary release

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

Provide environment information

$ yarn next info

    Operating System:
      Platform: darwin
      Arch: arm64
      Version: Darwin Kernel Version 21.2.0: Sun Nov 28 20:28:41 PST 2021; root:xnu-8019.61.5~1/RELEASE_ARM64_T6000
    Binaries:
      Node: 18.1.0
      npm: 8.8.0
      Yarn: 3.2.0
      pnpm: N/A
    Relevant packages:
      next: 12.1.6
      react: 18.1.0
      react-dom: 18.1.0

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Describe the Bug

This is actually a following of #33464.

Running yarn build && yarn export while using an updated exportPathMap in next.config.js leads to the creation of an extra 404.html.html file in export. It only happens if the object returned from exportPathMap doesn't have a key named /404.

next.js/packages/next/export/index.ts

Line 421 in e8a8220

if (

For example, my goal was to create each page in an index.html in a directory with the same name (ie. /about.html becomes /about/index.html):

When using this next.config.js, the export produces an extra 404.html.html file (ie. the bug happens)...

// next.config.js

module.exports = {
  exportPathMap: async function (defaultPathMap) {
    const pathMap = {};

    for (const [path, config] of Object.entries(defaultPathMap)) {
      if (path === "/") {
        pathMap[path] = config;
      } else {
        pathMap[`${path}/index`] = config;
      }
    }

    return pathMap;
  }
}

... while when using this one, it doesn't happen:

// next.config.js

module.exports = {
  exportPathMap: async function (defaultPathMap) {
    // 🚩the only difference is here, we spread the default pathMap
    const pathMap = { ...defaultPathMap };

    for (const [path, config] of Object.entries(defaultPathMap)) {
      if (path === "/") {
        pathMap[path] = config;
      } else {
        pathMap[`${path}/index`] = config;
      }
    }

    return pathMap;
  }
}

Expected Behavior

The 404.html.html page should not be created when using a custom exportPathMap in next.config.js.

To Reproduce

1. Open https://stackblitz.com/edit/nextjs-rxdeeq
2. Run npx next build && npx next export in the terminal
3. 404.html.html has been created in the out/ directory

[hanneslund]

I think #36827 fixed this already.

[mdubourg001]

@hanneslund I don't think this PR fixes the same issue. I understand why the export of /404.html is enforced, but I'm not sure the export of /404.html.html should be:

next.js/packages/next/export/index.ts

Line 426 in e8a8220

exportPathMap['/404'] = exportPathMap['/404.html'] = {

[SukkaW]

@mdubourg001

The issue should be solved by #36827 already. I have forked your stackblitz reproduction, and updated Next.js to the next@canary, and no 404.html.html is being exported: https://stackblitz.com/edit/nextjs-oujzwx

[github-actions]

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