`loadConfig` mutates the `imageConfigDefault` object

What version of Next.js are you using?

12.0.7

What version of Node.js are you using?

16.13.1

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

next start

Describe the Bug

Inside of assignDefaults function we merge and copy defaultConfig with the passed in config.

// https://github.com/vercel/next.js/blob/v12.0.7/packages/next/server/config.ts#L14
const result = { ...defaultConfig, ...config }

Then later mutates result by assigning sub-properties.

        // https://github.com/vercel/next.js/blob/v12.0.7/packages/next/server/config.ts#L205
        images.domains.push(new URL(config.assetPrefix).hostname)
      
      // https://github.com/vercel/next.js/blob/v12.0.7/packages/next/server/config.ts#L279
      images.loader = 'default'
      
      // https://github.com/vercel/next.js/blob/v12.0.7/packages/next/server/config.ts#L314
      images.path = `${result.basePath}${images.path}`

Because the merge and copy is shallow, when we mutate result.images, we are actually mutating the original object imageConfigDefault exported from https://github.com/vercel/next.js/blob/v12.0.7/packages/next/server/image-config.ts.

This could cause a number of issues, for example, in the case I’m running into, every time loadConfig is called the basePath property is pre-pended to imageConfigDefault.path.

console.log(imageConfigDefault.path)
// "/_next/image"

await loadConfig("phase-development-server", "./path/to/config/with/foo/base/path");
await loadConfig("phase-development-server", "./path/to/config/with/foo/base/path");
await loadConfig("phase-development-server", "./path/to/config/with/foo/base/path");

console.log(imageConfigDefault.path)
// "/foo/foo/foo/_next/image"

See the codesandbox for an example.

Expected Behavior

Calling loadConfig with a config that has a basePath should not mutate imageConfigDefault or defaultConfig.

To Reproduce

https://codesandbox.io/s/immutable-thunder-g3bj8?file=/server.js

[timneutkens]

So this is indeed unexpected, however loadConfig is only ever called once so this would indicate that you’re using internals in some way, what are you doing exactly? 🤔

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