Default useArrowFunction is wrong when using mocha

[BadIdeaException]

Within mocha function calls (e.g. describe(), test()) the useArrowFunction rule should be disabled.

Mocha binds this to the test context, so using an arrow function will lose access to this context. In fact, it is actively discouraged in their docs.

[manimovassagh]

You're right — Mocha binds this to the test context, and arrow functions capture the outer this, which breaks access to this.timeout(), this.retries(), etc.

Until the rule gets a built-in option to detect Mocha/other test framework contexts, you can work around it with an override in your biome.json:

{
  "overrides": [
    {
      "include": ["**/*.test.{js,ts}", "**/*.spec.{js,ts}", "**/test/**"],
      "linter": {
        "rules": {
          "style": {
            "useArrowFunction": "off"
          }
        }
      }
    }
  ]
}

This disables the rule only in test files while keeping it active everywhere else.

If you want something more targeted (only disabling it inside describe/it/test blocks rather than the whole file), that's not possible with Biome's current configuration — the override granularity is at the file level, not the AST level.

This would be a good candidate for a rule option, similar to how eslint-plugin-mocha handles it. The rule could accept a list of function names (e.g., describe, it, test, before, after, beforeEach, afterEach) whose callbacks should be excluded from the arrow function suggestion. If you want to push this forward, filing a feature request for an ignoredCallers or ignoredWrappers option on useArrowFunction would be the way to go.

[BadIdeaException]

Hey, thanks for showing me how to disable the rule for test files. I was having trouble with that - this is my first time giving biome a try (loving it so far!)

Not sure how you envision the ignoredCallers thing? I was more guessing when you detect mocha in the dependencies you default to disabling the rule like you showed me...

[manimovassagh]

Good catch. Mocha explicitly discourages arrow functions because it binds this to the test context (for things like this.timeout(), this.retries(), this.slow(), etc.), and arrow functions don't have their own this.

Until Biome adds framework-aware configuration for useArrowFunction, you can work around this by disabling the rule for your test files in biome.json:

{
  "overrides": [
    {
      "includes": ["**/*.test.{js,ts}", "**/*.spec.{js,ts}", "**/test/**"],
      "linter": {
        "rules": {
          "style": {
            "useArrowFunction": "off"
          }
        }
      }
    }
  ]
}

This keeps the rule active for your application code while disabling it where Mocha needs regular functions.

For a more targeted approach, you could also use inline suppression:

// biome-ignore lint/style/useArrowFunction: mocha binds this to test context
describe(function () {
  this.timeout(5000);
});

The overrides approach is cleaner though — set it once and forget it.