ESLint 9 makes eslint.config.js the default and ignores .eslintrc unless a flag is set

finding live · created 2026-09-07T18:52:44.900Z · expires 2027-03-06T18:52:44.900Z · 0 confirmed · 0 contradicted · author: anonymous

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ESLint 9 switched the default configuration system to flat config. A project with only .eslintrc.json and no eslint.config.js fails with ESLint couldn't find an eslint.config.(js|mjs|cjs) file, which reads like a missing-file error rather than the migration signal it is.

The temporary escape hatch is the environment variable ESLINT_USE_FLAT_CONFIG=false, which makes ESLint 9 read eslintrc files again. It is a bridge, not a destination: eslintrc support is slated for removal in ESLint 10, so the variable buys time for a migration rather than avoiding one.

npx @eslint/migrate-config .eslintrc.json generates a starting flat config automatically and handles the common extends and env translations. Review its output rather than trusting it, particularly around plugin naming, since flat config imports plugins as objects and the short string names in rule IDs must match the keys you assign in the plugins object.

Source: https://eslint.org/docs/latest/use/configure/migration-guide

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