ESLint is a popular linter for JavaScript. The ESLint integration also uses Prettier to enforce code formatting.

Run Locally

The mozlint integration of ESLint can be run using mach:

$ mach lint --linter eslint <file paths>

Alternatively, omit the --linter eslint and run all configured linters, which will include ESLint.

ESLint also supports the --fix option to autofix most errors raised from most of the rules.

See the Usage guide for more options.

Understanding Rules and Errors

Common Issues and How To Solve Them

My editor says that mozilla/whatever is unknown

  • Run ./mach eslint --setup, and restart your editor.

My editor doesn’t understand a new global I’ve just added (e.g. to a content file or head.js file)

  • This is a limitation which is a mixture of our ESLint setup and how we share globals across files.

  • Restarting your editor should pick up the new globals.

  • You can always double check via ./mach lint –linter eslint <file path> on the command line.

This code shouldn’t be linted

  • If it is a third-party piece of code, please add it to ThirdPartyPaths.txt.

  • If it is pre-generated file or intentionally invalid, please add it to .eslintignore

I’m using an es6 module

I have valid code that is failing the no-undef rule or can’t be parsed

  • Please do not add this to .eslintignore. Generally this can be fixed, if the following tips don’t help, please seek help.

  • If you are adding a new test directory, make sure its name matches one of the patterns in .eslintrc.js. If you really can’t match those, then you may need to add a separate test specific .eslintrc.js file (example).

  • If you are writing a frame script you may need to tell ESLint to use the frame script environment:

    • /* eslint-env mozilla/frame-script */

  • If you are writing a worker, then you may need to use the worker or chrome-worker environment:

    • /* eslint-env worker */

    • /* eslint-env mozilla/chrome-worker */

  • I use Services.scriptloader.loadSubScript:

    • /* import-globals-from relative/path/to/file.js


The global configuration file lives in topsrcdir/.eslintrc. This global configuration can be overridden by including an .eslintrc in the appropriate subdirectory. For an overview of the supported configuration, see ESLint’s documentation.

Please keep differences in rules across the tree to a minimum. We want to be consistent to make it easier for developers.