Eslint Migrate Options

Purpose

Use this skill when a Biome lint rule already exists and biome migrate eslint should preserve more than just the rule severity.

This skill is specifically for cases where an ESLint rule has options that need to be:

• deserialized from ESLint config
• translated into Biome rule options
• wired into the migrate pipeline
• tested through migrator spec fixtures without depending on CLI tests

Do not use this skill for severity-only migrations. Those are usually covered by the generated rule mapping in eslint_any_rule_to_biome.rs.

Before You Edit

Confirm these points first:

1. The target Biome rule already exists and already has its own options type in crates/biome_rule_options/src/. 2. The Biome rule metadata already declares the ESLint source rule, so severity-only migration exists or can be generated. 3. The ESLint rule really has user-facing options worth preserving. 4. You have checked the ESLint rule docs or source so you know the exact option shape, defaults, and any plugin-specific quirks.

If any of those are missing, fix that first before adding a migrator.

Mental Model

The migrate pipeline has two layers:

1. Generated severity mapping: eslint_any_rule_to_biome.rs 2. Hand-written option migration: plugin-specific structs plus a custom arm in migrate_eslint_rule()

The generated file already handles the common case:

{
  "some-rule": "error"
}

Add a custom migrator only when a config like this should keep its options:

{
  "some-rule": ["error", { "someOption": true }]
}

Key Files

Use the plugin-specific file that matches the source ESLint rule. Keep option structs close to similar migrators so future edits stay discoverable.

Recommended Workflow

Step 1: Inspect an Existing Migrator First

Before writing anything new, find a nearby rule that already migrates options. Reuse its shape if the target rule is in the same plugin or has the same Biome configuration type (RuleConfiguration vs RuleFixConfiguration).

This saves time and helps match the patterns already used in migrate_eslint_rule().

Step 2: Model the ESLint Options Exactly

Add structs in the correct plugin file. Match ESLint's option payload shape, not Biome's.

use biome_deserialize_macros::Deserializable;

#[derive(Clone, Debug, Default, Deserializable)]
pub(crate) struct EslintMyRuleOptions {
    some_option: Option<u8>,
    another_option: bool,
    nested: EslintMyRuleNestedOptions,
}

#[derive(Clone, Debug, Default, Deserializable)]
pub(crate) struct EslintMyRuleNestedOptions {
    threshold: Option<u8>,
}

Guidelines:

• Use snake_case Rust field names; Deserializable handles camelCase JSON keys.
• Use Option<T> for fields that can be omitted.
• Keep unsupported ESLint fields in the struct if they appear in the config shape; ignore them later during conversion.
• Prefer mirroring the real JSON nesting instead of flattening early.

Step 3: Convert ESLint Options Into Biome Options

Implement From<Eslint...Options> for biome_rule_options::... in the same plugin file.

impl From<EslintMyRuleOptions> for my_rule::MyRuleOptions {
    fn from(value: EslintMyRuleOptions) -> Self {
        Self {
            some_option: value.some_option,
            different_name: Some(value.another_option),
            threshold: value.nested.threshold,
        }
    }
}

Focus on semantic mapping, not field-for-field copying:

• rename concepts when ESLint and Biome use different names
• drop unsupported knobs deliberately
• preserve defaults only when they match Biome's behavior
• add small helper functions when the conversion needs filtering or normalization

If an ESLint option should only be emitted when at least one nested field is set, use a helper that returns Option<_> rather than constructing empty Biome option objects.

Step 4: Add a Typed Rule Variant

In eslint_eslint.rs, add a Rule enum variant using RuleConf<T>:

pub(crate) enum Rule {
    // ...
    MyPluginMyRule(RuleConf<eslint_my_plugin::EslintMyRuleOptions>),
}

Then update both of these places:

• Rule::name() so the variant returns the ESLint rule name
• Rules::deserialize so the ESLint rule string deserializes into your typed variant before the catch-all fallback

Example:

Self::MyPluginMyRule(_) => Cow::Borrowed("my-plugin/my-rule"),

"my-plugin/my-rule" => {
    if let Some(conf) = RuleConf::deserialize(ctx, &value, name) {
        result.insert(Rule::MyPluginMyRule(conf));
    }
}

Order matters in Rules::deserialize: put the explicit match before the fallback rule_name => arm.

Step 5: Wire the Rule Into migrate_eslint_rule()

Add a match arm in crates/biome_cli/src/execute/migrate/eslint_to_biome.rs.

Always call migrate_eslint_any_rule() first. It handles severity tracking, unsupported-rule reporting, and deduplication.

Pick the configuration type that matches the Biome rule:

• RuleFixConfiguration::WithOptions for fixable rules
• RuleConfiguration::WithOptions for non-fixable rules

Typical fixable rule pattern:

eslint_eslint::Rule::MyPluginMyRule(conf) => {
    if migrate_eslint_any_rule(rules, &name, conf.severity(), opts, results) {
        let group = rules.style.get_or_insert_with(Default::default);
        if let SeverityOrGroup::Group(group) = group {
            group.my_biome_rule = Some(biome_config::RuleFixConfiguration::WithOptions(
                biome_config::RuleWithFixOptions {
                    level: conf.severity().into(),
                    fix: None,
                    options: conf.option_or_default().into(),
                },
            ));
        }
    }
}

Typical non-fixable rule pattern:

eslint_eslint::Rule::MyPluginMyRule(conf) => {
    if migrate_eslint_any_rule(rules, &name, conf.severity(), opts, results) {
        let group = rules.style.get_or_insert_with(Default::default);
        if let SeverityOrGroup::Group(group) = group {
            group.my_biome_rule = Some(biome_config::RuleConfiguration::WithOptions(
                biome_config::RuleWithOptions {
                    level: conf.severity().into(),
                    options: conf.option_or_default().into(),
                },
            ));
        }
    }
}

Replace rules.style with the correct group (a11y, complexity, correctness, nursery, performance, security, style, suspicious).

Step 6: Choose the Right RuleConf Access Pattern

Do not force every migrator into the same shape. The current codebase uses different access patterns depending on the ESLint rule schema.

Use the one that matches the source rule:

• conf.option_or_default() when the rule has one options object and severity-only configs should fall back to defaults
• if let RuleConf::Option(severity, rule_options) = conf when the migration should only attach options if the user explicitly provided the object
• conf.into_vec() when the rule uses array-style payloads that need custom aggregation or normalization

If unsure, inspect an existing migrator with a similar ESLint schema and copy that pattern.

Common Pitfalls

• Adding a custom migrator when severity-only migration was enough
• Modeling the Biome options instead of the ESLint JSON shape
• Forgetting to update both Rule::name() and Rules::deserialize
• Putting the deserialization arm after the fallback arm
• Writing a custom match arm but skipping migrate_eslint_any_rule()
• Using RuleFixConfiguration for a rule that is not fixable, or the inverse
• Emitting empty option objects that change semantics compared with the default Biome config
• Ignoring ESLint fields during deserialization by leaving them out of the struct, causing valid configs to fail to deserialize

Worked Example

unicorn/numeric-separators-style is a good reference because the names do not line up perfectly.

ESLint uses number; Biome uses decimal. ESLint also exposes onlyIfContainsSeparator, which Biome does not support, so the migrator ignores it.

#[derive(Clone, Debug, Default, Deserializable)]
pub(crate) struct NumericSeparatorsStyleOptions {
    number: EslintNumericSeparatorTypeOptions,
    binary: EslintNumericSeparatorTypeOptions,
    octal: EslintNumericSeparatorTypeOptions,
    hexadecimal: EslintNumericSeparatorTypeOptions,
}

#[derive(Clone, Debug, Default, Deserializable)]
pub(crate) struct EslintNumericSeparatorTypeOptions {
    minimum_digits: Option<u8>,
    group_length: Option<u8>,
}

impl From<NumericSeparatorsStyleOptions>
    for use_numeric_separators::UseNumericSeparatorsOptions
{
    fn from(value: NumericSeparatorsStyleOptions) -> Self {
        Self {
            binary: some_if_set(value.binary),
            octal: some_if_set(value.octal),
            decimal: some_if_set(value.number),
            hexadecimal: some_if_set(value.hexadecimal),
        }
    }
}

fn some_if_set(
    options: EslintNumericSeparatorTypeOptions,
) -> Option<use_numeric_separators::NumericLiteralSeparatorOptions> {
    if options.minimum_digits.is_some() || options.group_length.is_some() {
        Some(options.into())
    } else {
        None
    }
}

This is the pattern to follow when:

• ESLint names differ from Biome names
• nested objects may be partially unset
• empty nested config should collapse to None

Testing Checklist

At minimum, verify all of these:

1. Severity-only ESLint config still migrates correctly. 2. ESLint config with options produces the expected Biome options. 3. Unsupported ESLint knobs do not break deserialization. 4. Empty or partially specified nested options do not emit incorrect Biome config. 5. If Biome's defaults differ from ESLint's, severity-only configs should not emit Biome options that change behavior.

Use the migrator spec fixtures in crates/biome_cli/tests/specs/migrate_eslint/ for custom migrators.

• Add one fixture file per case.
• Keep the fixture focused on eslint input and pre-migration biome config input.
• Let the generated test runner in eslint_to_biome.rs discover the file and write the adjacent .snap.new.
• Add fixtures for every relevant option shape, including severity-only configs when defaults differ between ESLint and Biome.
• After inspecting snapshot differences, use cargo insta accept to accept valid new snapshots, or cargo insta reject to reject invalid ones and keep iterating.

Useful commands:

cargo check -p biome_cli
cargo test -p biome_cli migrate_eslint

When the rule itself has analyzer behavior tied to the options, run targeted analyzer tests too:

cargo test -p biome_js_analyze my_rule_name

Review Checklist

Before finishing, confirm:

• the typed Rule variant exists
• Rule::name() returns the exact ESLint rule name
• Rules::deserialize has an explicit arm before the fallback
• the plugin-specific ESLint option structs match the real ESLint schema
• the From impl maps semantics correctly, not just names mechanically
• migrate_eslint_any_rule() is still called first
• the chosen Biome rule group and configuration type are correct
• migrator spec fixtures cover both severity-only and option-bearing configs when relevant

References

• crates/biome_cli/src/execute/migrate/
• crates/biome_cli/src/execute/migrate/eslint_eslint.rs
• crates/biome_cli/src/execute/migrate/eslint_to_biome.rs
• crates/biome_cli/src/execute/migrate/eslint_any_rule_to_biome.rs
• crates/biome_rule_options/src/
• xtask/codegen/src/generate_migrate_eslint.rs