What are the main ESLint rule categories?

Since ESLint 8 the core rules are grouped into three types: Possible Problems, which catch likely bugs; Suggestions, which improve code without flagging clear errors; and Layout & Formatting, which concern whitespace and style. Older docs used Possible Errors, Best Practices, Variables, Stylistic and ES6.

What does the fixable flag mean?

A fixable rule can be auto-corrected when you run eslint --fix, with no manual edit needed. Many Layout & Formatting and some Suggestion rules are fixable; most Possible Problems rules are not because the safe fix is ambiguous.

What is the recommended preset?

eslint:recommended is a curated subset of core rules that catch common, high-confidence problems. Enabling it via extends gives you a sensible baseline; rules marked recommended in this list are the ones it turns on.

Are formatting rules still in ESLint core?

They remain but are frozen and deprecated for new work. ESLint recommends delegating formatting to Prettier or the separate @stylistic plugin, and reserving ESLint for logic and correctness rules.

How do I enable a single rule?

Add it to the rules object in your config with a severity: 'off', 'warn' or 'error', optionally with options. For example, 'no-unused-vars': 'error' turns the rule on at error level.

ESLint Rule Categories Reference

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ESLint rules grouped by category

ESLint ships dozens of built-in core rules. Since ESLint 8 they are organized into three categories by intent, and each rule carries two important flags: whether eslint --fix can auto-correct it, and whether the eslint:recommended preset enables it. This searchable reference lists representative core rules per category so you can find what a rule does and how to wire it up.

How it works

A rule is enabled in your config by name with a severity:

// eslint.config.js (flat config)
export default [
  {
    rules: {
      "no-unused-vars": "error",
      "eqeqeq": ["error", "always"],
      "prefer-const": "warn"
    }
  }
];

Rules in the Possible Problems category catch likely bugs (no-undef, no-unreachable). Suggestions rules nudge toward better patterns (prefer-const, eqeqeq). Layout & Formatting rules govern whitespace and are mostly fixable but now deprecated in favour of Prettier or @stylistic.

Tips and notes

Start from eslint:recommended, then add rules as your team agrees on them.
Prefer --fix for the fixable Suggestion and Layout rules to save manual work.
Move pure formatting to Prettier; keep ESLint focused on correctness.
A rule’s severity can be off, warn or error; only error fails CI.