denoland/deno · error · Error

Linter plugin rules must be an object

Error message

${prefix}Linter plugin rules must be an object

What it means

A Deno lint plugin must expose its rules as an object: `rules` maps rule names (like "no-foo") to rule objects with a create(ctx) method. installPlugin in cli/js/40_lint.js throws this error when typeof plugin.rules !== "object" — i.e. `rules` is missing, or is a string, number, boolean, or function. The check is prefixed with the plugin specifier when available.

Solutions

1. Add a `rules` property that is a plain object: `rules: { "no-foo": { create(ctx) { ... } } }`.
2. If you export the rules map separately, wrap it: `export default { name: "my-plugin", rules };`.
3. Make sure the default export is the plugin object itself, not a function or the rules map.
4. Add a smoke test that imports the plugin and asserts typeof plugin.rules === "object" && plugin.rules !== null.

Example fix

// before
const rules = {
  "no-foo": { create(ctx) { return {}; } },
};
export default rules;

// after
const rules = {
  "no-foo": { create(ctx) { return {}; } },
};
export default { name: "my-plugin", rules };

function assertPluginShape(plugin) {
  if (typeof plugin !== "object" || plugin === null) throw new Error("plugin must be an object");
  if (typeof plugin.name !== "string") throw new Error("plugin.name must be a string");
  if (typeof plugin.rules !== "object" || plugin.rules === null) {
    throw new Error("plugin.rules must be a non-null object");
  }
}

/** @param {unknown} p @returns {p is Deno.lint.Plugin} */
function isValidLintPlugin(p) {
  return typeof p === "object" && p !== null &&
    typeof p.name === "string" && /^[a-z-]+$/.test(p.name) &&
    typeof p.rules === "object" && p.rules !== null;
}

When it happens

Trigger: Installing a plugin whose default export is `{ name: "my-plugin" }` (no `rules` key), exports the rules object directly instead of wrapping it, or sets `rules` to a string/number/function. Note that `rules: null` and arrays pass the typeof check and fail later (Object.keys on null throws), so the real requirement is a non-null plain object.

Common situations: Exporting `export default rules;` instead of `export default { name, rules };` when porting an ESLint plugin; forgetting the rules key in a minimal plugin skeleton; `rules` accidentally overwritten by a build step.

Related errors

• Linter plugin name must not have consequtive hyphens.
• Linter plugin name must only contain lowercase letters…
• Linter plugin name must start and end with a lowercase…
• Destroy hook of " " errored
• Empty node

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/eb1a3863988fdafe. Report an issue: GitHub.

Appendix: source

Thrown at cli/js/40_lint.js:504

    throw new Error(`${prefix}Linter plugin name must be a string`);
  }
  if (!/^[a-z-]+$/.test(plugin.name)) {
    throw new Error(
      `${prefix}Linter plugin name must only contain lowercase letters (a-z) or hyphens (-).`,
    );
  }
  if (plugin.name.startsWith("-") || plugin.name.endsWith("-")) {
    throw new Error(
      `${prefix}Linter plugin name must start and end with a lowercase letter.`,
    );
  }
  if (plugin.name.includes("--")) {
    throw new Error(
      `${prefix}Linter plugin name must not have consequtive hyphens.`,
    );
  }
  if (typeof plugin.rules !== "object") {
    throw new Error(`${prefix}Linter plugin rules must be an object`);
  }
  if (state.installedPlugins.has(plugin.name)) {
    throw new Error(`Linter plugin ${plugin.name} has already been registered`);
  }
  state.plugins.push(plugin);
  state.installedPlugins.add(plugin.name);

  return {
    name: plugin.name,
    ruleNames: Object.keys(plugin.rules),
  };
}

/**
 * @param {AstContext} ctx
 * @param {number} idx
 * @returns {FacadeNode | null}
 */

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