Lint Rule Development

Purpose

Use this skill when creating new lint rules or assist actions for Biome. It provides scaffolding commands, implementation patterns, testing workflows, and documentation guidelines.

Prerequisites

1. Install required tools: just install-tools
2. Ensure cargo, just, and pnpm are available
3. Read crates/biome_analyze/CONTRIBUTING.md for in-depth concepts

Common Workflows

Create a New Lint Rule

Generate scaffolding for a JavaScript lint rule:

just new-js-lintrule useMyRuleName

For other languages:

just new-css-lintrule myRuleName
just new-json-lintrule myRuleName
just new-graphql-lintrule myRuleName

This creates a file in crates/biome_js_analyze/src/lint/nursery/use_my_rule_name.rs

Implement the Rule

Basic rule structure (generated by scaffolding):

use biome_analyze::{context::RuleContext, declare_lint_rule, Rule, RuleDiagnostic};
use biome_js_syntax::JsIdentifierBinding;
use biome_rowan::AstNode;

declare_lint_rule! {
    /// Disallows the use of prohibited identifiers.
    pub UseMyRuleName {
        version: "next",
        name: "useMyRuleName",
        language: "js",
        recommended: false,
    }
}

impl Rule for UseMyRuleName {
    type Query = Ast<JsIdentifierBinding>;
    type State = ();
    type Signals = Option<Self::State>;
    type Options = ();

    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let binding = ctx.query();

        // Check if identifier matches your rule logic
        if binding.name_token().ok()?.text() == "prohibited_name" {
            return Some(());
        }

        None
    }

    fn diagnostic(ctx: &RuleContext<Self>, _state: &Self::State) -> Option<RuleDiagnostic> {
        let node = ctx.query();
        Some(
            RuleDiagnostic::new(
                rule_category!(),
                node.range(),
                markup! {
                    "Avoid using this identifier."
                },
            )
            .note(markup! {
                "This identifier is prohibited because..."
            }),
        )
    }
}

Using Semantic Model

For rules that need binding analysis:

use crate::services::semantic::Semantic;

impl Rule for MySemanticRule {
    type Query = Semantic<JsReferenceIdentifier>;

    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let node = ctx.query();
        let model = ctx.model();

        // Check if binding is declared
        let binding = node.binding(model)?;

        // Get all references to this binding
        let all_refs = binding.all_references(model);

        // Get only read references
        let read_refs = binding.all_reads(model);

        // Get only write references
        let write_refs = binding.all_writes(model);

        Some(())
    }
}

Add Code Actions (Fixes)

To provide automatic fixes:

use biome_analyze::FixKind;

declare_lint_rule! {
    pub UseMyRuleName {
        version: "next",
        name: "useMyRuleName",
        language: "js",
        recommended: false,
        fix_kind: FixKind::Safe, // or FixKind::Unsafe
    }
}

impl Rule for UseMyRuleName {
    fn action(ctx: &RuleContext<Self>, _state: &Self::State) -> Option<JsRuleAction> {
        let node = ctx.query();
        let mut mutation = ctx.root().begin();

        // Example: Replace the node
        mutation.replace_node(
            node.clone(),
            make::js_identifier_binding(make::ident("replacement"))
        );

        Some(JsRuleAction::new(
            ctx.metadata().action_category(ctx.category(), ctx.group()),
            ctx.metadata().applicability(),
            markup! { "Use 'replacement' instead" }.to_owned(),
            mutation,
        ))
    }
}

Quick Testing

Use the quick test for rapid iteration:

// In crates/biome_js_analyze/tests/quick_test.rs
// Uncomment #[ignore] and modify:

const SOURCE: &str = r#"
const prohibited_name = 1;
"#;

let rule_filter = RuleFilter::Rule("nursery", "useMyRuleName");

Run the test:

cd crates/biome_js_analyze
cargo test quick_test -- --show-output

Create Snapshot Tests

Create test files in tests/specs/nursery/useMyRuleName/:

tests/specs/nursery/useMyRuleName/
├── invalid.js          # Code that triggers the rule
├── valid.js            # Code that doesn't trigger the rule
└── options.json        # Optional rule configuration

Every test file must start with a top-level comment declaring whether it expects diagnostics. The test runner enforces this — see the testing-codegen skill for full rules. The short version:

valid.js — comment is mandatory (test panics without it):

/* should not generate diagnostics */
const x = 1;
const y = 2;

invalid.js — comment is strongly recommended (also enforced when present):

/* should generate diagnostics */
const prohibited_name = 1;
const another_prohibited = 2;

Run snapshot tests:

just test-lintrule useMyRuleName

Review snapshots:

cargo insta review

Generate Analyzer Code

During development, use the lightweight codegen commands:

just gen-rules          # Updates rule registrations in *_analyze crates
just gen-configuration  # Updates configuration schemas

These generate enough code to compile and test your rule without errors.

For full codegen (migrations, schema, bindings, formatting), run:

just gen-analyzer

Note: The CI autofix job runs gen-analyzer automatically when you open a PR, so running it locally is optional.

Format and Lint

Before committing:

just f  # Format code
just l  # Lint code

Adding Configurable Options

When a rule needs user-configurable behavior, add options via the biome_rule_options crate. For the full reference (merge strategies, design guidelines, common patterns), see references/OPTIONS.md.

Quick workflow:

Step 1. Define the options type in biome_rule_options/src/<snake_case_rule_name>.rs:

use biome_deserialize_macros::{Deserializable, Merge};
use serde::{Deserialize, Serialize};

#[derive(Debug, Default, Clone, Serialize, Deserialize, Deserializable, Merge)]
#[cfg_attr(feature = "schema", derive(schemars::JsonSchema))]
#[serde(rename_all = "camelCase", deny_unknown_fields, default)]
pub struct UseMyRuleNameOptions {
    #[serde(skip_serializing_if = "Option::is_none")]
    pub behavior: Option<MyBehavior>,
}

Step 2. Wire it into the rule:

use biome_rule_options::use_my_rule_name::UseMyRuleNameOptions;

impl Rule for UseMyRuleName {
    type Options = UseMyRuleNameOptions;

    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let options = ctx.options();
        let behavior = options.behavior.unwrap_or_default();
        // ...
    }
}

Step 3. Test with options.json in the test directory (see references/OPTIONS.md for examples).

Step 4. Run codegen: just gen-rules && just gen-configuration

Key rules:

• All fields must be Option<T> for config merging to work
• Use Box<[Box<str>]> instead of Vec<String> for collection fields
• Use #[derive(Merge)] for simple cases, implement Merge manually for collections
• Only add options when truly needed (conflicting community preferences, multiple valid interpretations)

Tips

• Rule naming: Use no* prefix for rules that forbid something (e.g., noVar), use* for rules that mandate something (e.g., useConst)
• Nursery group: All new rules start in the nursery group
• Semantic queries: Use Semantic<Node> query when you need binding/scope analysis
• Multiple signals: Return Vec<Self::State> or Box<[Self::State]> to emit multiple diagnostics
• Safe vs Unsafe fixes: Mark fixes as Unsafe if they could change program behavior
• Check for globals: Always verify if a variable is global before reporting it (use semantic model)
• Error recovery: When navigating CST, use .ok()? pattern to handle missing nodes gracefully
• Testing arrays: Use .jsonc files with arrays of code snippets for multiple test cases

Common Query Types

// Simple AST query
type Query = Ast<JsVariableDeclaration>;

// Semantic query (needs binding info)
type Query = Semantic<JsReferenceIdentifier>;

// Multiple node types (requires declare_node_union!)
declare_node_union! {
    pub AnyFunctionLike = AnyJsFunction | JsMethodObjectMember | JsMethodClassMember
}
type Query = Semantic<AnyFunctionLike>;

References

• Full guide: crates/biome_analyze/CONTRIBUTING.md
• Rule examples: crates/biome_js_analyze/src/lint/
• Semantic model: Search for Semantic< in existing rules
• Testing guide: Main CONTRIBUTING.md testing section