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Biome is a linter and formatter in one binary. It covers the ground that ESLint and Prettier cover together, with one configuration file and one dependency. Biome runs beside the Extension.js build, never inside it. Extension.js does not read biome.json, does not run Biome, and does not copy the configuration into the packaged output.

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When Biome is a good fit

  • You want one tool and one config file instead of a linter plus a formatter.
  • You are starting a project and have no ESLint rule set to carry over.
  • You want checks that finish fast enough to run on every save.

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Biome capabilities

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There is no Biome template

Extension.js ships configuration templates for ESLint, Prettier, and Stylelint. It ships none for Biome. Scaffold any template and add Biome yourself with the steps below.

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Add Biome to an extension

Install the package:
Create the configuration file:
That writes a biome.json at the project root. The generated file already keeps Biome out of the build output:
biome.json
Run the checks:
Apply the fixes that Biome can make on its own:

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Add the scripts

Biome writes no scripts into package.json. Add them next to the Extension.js scripts:
package.json
biome ci is the continuous integration form. It reports the same findings as check and never edits a file.

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Ignore the generated type file

A TypeScript project carries an extension-env.d.ts file that Extension.js rewrites on every dev and build run. Formatting it is wasted work, because the next run replaces it. Exclude it:
biome.json
Read Types for the chrome and browser APIs for what that file does.

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Migrating from ESLint or Prettier

Biome reads an existing configuration and converts what it can:
Remove the old dependencies once the output satisfies you. Nothing in the Extension.js build depends on either tool.

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What the build does with biome.json

Nothing. The build reads a closed set of root files: manifest.json, package.json, extension.config.js, tsconfig.json, the PostCSS and Tailwind configs, and the .env family. A biome.json file is never read, never watched, and never emitted into dist/. One packaging detail is worth knowing. extension build --zip-source archives the whole project folder, so biome.json lands in the source archive. That is usually what you want for a store review that asks for sources.

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Best practices

  • Choose Biome or the ESLint and Prettier pair. Running both formats the same files twice, with different results.
  • Keep dist excluded. The generated bundle is not yours to format.
  • Run biome ci in continuous integration, not biome check --write. A CI job should report, not rewrite.

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Next steps