facebook/flow · error · Error

flow-remove-types: includes and excludes must be RegExp or p

Error message

flow-remove-types: includes and excludes must be RegExp or path strings. Got: ${pattern}

What it means

flow-remove-types/register converts each pattern in includes/excludes into a RegExp: strings get '.' escaped and '*' widened to '.*' (and are prefixed with a leading '/'), and anything with a .test function is used as-is. This error is the validation branch for patterns that are neither strings nor RegExp-likes, thrown at startup of the require hook rather than at transform time.

Source

Thrown at packages/flow-remove-types/register.js:73

// Given a null | string | RegExp | any, returns null | Regexp or throws a
// more helpful error.
function regexpPattern(pattern) {
  if (!pattern) {
    return pattern;
  }
  // A very simplified glob transform which allows passing legible strings like
  // "myPath/*.js" instead of a harder to read RegExp like /\/myPath\/.*\.js/.
  if (typeof pattern === 'string') {
    pattern = pattern.replace(/\./g, '\\.').replace(/\*/g, '.*');
    if (pattern[0] !== '/') {
      pattern = '/' + pattern;
    }
    return new RegExp(pattern);
  }
  if (typeof pattern.test === 'function') {
    return pattern;
  }
  throw new Error(
    'flow-remove-types: ' +
      'includes and excludes must be RegExp or path strings. Got: ' +
      pattern,
  );
}

View on GitHub (pinned to d1341dac89)

Solutions

Make every entry a path string like 'src/*.js' or a RegExp such as new RegExp('/src/.*[.]js$')
Wrap the pattern list in an explicit array so a bare string is not iterated per character
Validate config types at load time (e.g. a zod or joi schema) before handing them to the register hook

Example fix

// before
require('flow-remove-types/register')({includes: 'lib/*.js'}); // string iterated -> junk entries

// after
require('flow-remove-types/register')({includes: ['lib/*.js']});

Defensive patterns

Strategy: type-guard

Validate before calling

const isPattern = (p) => typeof p === 'string' || (p != null && typeof p.test === 'function');
function assertPatterns(list) {
  for (const p of list) {
    if (!isPattern(p)) throw new TypeError('Pattern must be string or RegExp, got ' + typeof p);
  }
}

Type guard

const isValidPattern = (p) =>
  typeof p === 'string' || (p != null && typeof p === 'object' && typeof p.test === 'function');

Prevention

Always pass includes/excludes as arrays of strings or RegExps; never a bare string or nested arrays
Validate env- or JSON-sourced config values with a schema before the register hook reads them
Remember string patterns are matched against a leading '/', so write 'src/*.js' rather than a bare glob

When it happens

Trigger: Passing a number, boolean, null, array, or a plain object (e.g. a micromatch or parse-glob result) in the includes or excludes array of require('flow-remove-types/register')({includes: [...]}) or the equivalent CLI flags.

Common situations: Config parsed from JSON, YAML, or env vars where a pattern ends up typed as a number or list; passing glob objects from another library; typos like includes: 'src/*.js' where a bare string is iterated char-by-char, producing junk patterns that eventually hit this throw.

Related errors

AI-assisted analysis of facebook/flow@d1341dac89 (2026-08-17). Data as JSON: /api/errors/80935009ba403946. Report an issue: GitHub.