Fixes#40133
Fixes a small regression introduced in #39872. We should be able to detect if a non-empty `package.json#eslintConfig` property is present.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
We shouldn't be warning when no eslint config file is present as it's valid not to use eslint. The warning is still shown if an empty eslint config file is added as this gives intent to using eslint.
x-ref: [slack thread](https://vercel.slack.com/archives/CGU8HUTUH/p1661268593890619?thread_ts=1661266342.496699&cid=CGU8HUTUH)
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
This PR introduces an improved developer experience when `next lint` is run for the first time.
### Current behavior
`eslint-config-next` is a required package that must be installed before proceeding with `next lint` or `next build`:
Although this has helped many developers start using the new ESLint config, this has also resulted in a few issues:
- Users are required to install the full config (`eslint-config-next`) even if they do not use it or use the Next.js plugin directly (`eslint-plugin-next`).
- #26348
- There's some confusion on why `eslint-config-next` needs to be installed or how it should be used instead of `eslint-plugin-next`.
- #26574
- #26475
- #26438
### New behavior
Instead of enforcing `eslint-config-next` as a required package, this PR prompts the user by asking what config they would like to start. This happens when `next lint` is run for the first time **and** if no ESLint configuration is detected in the application.
<img src="https://user-images.githubusercontent.com/12476932/124331177-e1668a80-db5c-11eb-8915-38d3dc20f5d4.gif" width="800" />
- The CLI will take care of installing `eslint` or `eslint-config-next` if either is not already installed
- Users now have the option to choose between a strict configuration (`next/core-web-vitals`) or just the base configuration (`next`)
- For users that decide to create their own ESLint configuration, or already have an existing one, **installing `eslint-config-next` will not be a requirement for `next lint` or `next build` to run**. A warning message will just show if the Next.js ESLint plugin is not detected in an ESLint config.
<img width="682" alt="Screen Shot 2021-06-25 at 3 02 12 PM" src="https://user-images.githubusercontent.com/12476932/123473329-6cc4a680-d5c6-11eb-9a57-d5c0b89a2732.png">
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In addition, this PR also:
- Fixes#26348
- Updates documentation to make it more clear what approach to take for new and existing ESLint configurations
