denoland/deno · error · Error

${prefix}Linter plugin name must only contain lowercase lett

Error message

${prefix}Linter plugin name must only contain lowercase letters (a-z) or hyphens (-).

What it means

Thrown by installPlugin() in cli/js/40_lint.js when a Deno lint plugin's `name` fails the /^[a-z-]+$/ check. Plugin names are used to build rule ids like `plugin-name/rule-name`, so they must be strictly lowercase ASCII letters and single hyphens; the regex also rejects an empty name. When the plugin came from a `lint.plugins` entry in the config, the message is prefixed with the specifier so you know which file is at fault.

Source

Thrown at cli/js/40_lint.js:489

/**
 * @param {Deno.lint.Plugin} plugin
 * @param {string} [specifier] The specifier the plugin was loaded from, if any.
 */
function installPlugin(plugin, specifier) {
  // When the plugin was loaded from a specifier (i.e. `lint.plugins` in the
  // config), prefix validation errors with it so the user knows which plugin
  // is misbehaving instead of getting an anonymous error.
  const prefix = typeof specifier === "string"
    ? `Failed to load lint plugin '${specifier}': `
    : "";
  if (typeof plugin !== "object") {
    throw new Error(`${prefix}Linter plugin must be an object`);
  }
  if (typeof plugin.name !== "string") {
    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`);

View on GitHub (pinned to 89f33cbef2)

Solutions

Rename the plugin to lowercase letters and single hyphens, e.g. "my-plugin"
If the name came from a config-loaded plugin, check the 'Failed to load lint plugin \'<specifier>\':' prefix to find the offending file
Keep the name non-empty, starting/ending with a letter, with no consecutive hyphens (the following checks also apply)

Example fix

// before
export default {
  name: "myLintPlugin",
  rules: { /* ... */ },
};

// after
export default {
  name: "my-lint-plugin",
  rules: { /* ... */ },
};

Defensive patterns

Strategy: validation

Validate before calling

const NAME_RE = /^[a-z]+(?:-[a-z]+)*$/; // charset + edges + no doubles
function assertValidPluginName(name) {
  if (typeof name !== "string") throw new TypeError("plugin.name must be a string");
  if (!NAME_RE.test(name)) {
    throw new TypeError(
      `invalid plugin name '${name}': use lowercase letters and single inner hyphens`,
    );
  }
}
// run before registering / shipping the plugin
assertValidPluginName(plugin.name);

Type guard

function isValidLintPluginName(name) {
  return typeof name === "string" && /^[a-z]+(?:-[a-z]+)*$/.test(name);
}

Prevention

Derive plugin names from the package name early and never hand-edit them per rule
Add a unit test that validates your plugin object's shape (name + rules) so mis-edits fail in your CI, not in users' lint runs
Run `deno lint` once on a scratch file as a smoke test after any plugin edit

When it happens

Trigger: A plugin object whose `name` contains uppercase letters ("myPlugin"), digits ("plugin2"), underscores ("my_plugin"), whitespace, non-ASCII characters, or is the empty string "". Reached via `lint.plugins` in deno.json or programmatic plugin installation before any linting runs.

Common situations: Porting an ESLint plugin to a Deno lint plugin and keeping a camelCase or snake_case name; copying a plugin file and renaming it with a version suffix like "my-plugin-2"; forgetting the name field and defaulting it to "".

Related errors

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/bdb58f451906a0e6. Report an issue: GitHub.