Change Request: Allow manually opting into flat config without an `eslint.config.js`

ESLint version

v8.24.0

What problem do you want to solve?

Right now you have to create an eslint.config.js file to use flat config. It would be nice if flat configs could be passed on the command line using the -c or --config argument. The docs currently make it sound like it should work, but it doesn’t.

What do you think is the correct solution?

I can think of two options:

1. A CLI argument like --use-flat-config that would make ESLint treat the config as a flat config
2. Assume a config is a flat config if it’s ESM because the legacy config files can’t be ESM

I think I prefer option 1 because option 2 doesn’t support CJS flat configs, which I think are allowed?

Participation

• I am willing to submit a pull request for this change.

Additional comments

No response

[mdjermanovic]

I’m not opposed to adding an environment variable either.

Do we want it to also include an option to opt-out of flat config even if eslint.config.js is present?

• ESLINT_USE_FLAT_CONFIG=true - forces flat config mode regardless of the presence of eslint.config.js (--config would be loaded as a flat config file; it would error if there is no eslint.config.js and --config was not specified).
• ESLINT_USE_FLAT_CONFIG=false - forces eslintrc mode regardless of the presence of eslint.config.js.
• Not defined or any other value - works the same as now, presence of eslint.config.js determines the mode.

[nzakas]

Do we want it to also include an option to opt-out of flat config even if eslint.config.js is present?

That seems like a good idea. It would certainly make compatibility testing a lot easier.

I like the proposal so marking as accepted.

@TomerAberbach would you like to submit a PR for this?