dmtrKovalenko/fff · error

patterns array must have at least 1 element

Error message

patterns array must have at least 1 element

What it means

multiGrep validates that the patterns option is a non-empty array before crossing the FFI boundary, because the Rust side expects at least one newline-joined pattern. An empty or missing patterns array would produce a useless or unsafe call into the native library, so it is rejected in JS with this error.

Solutions

1. Check options.patterns before calling and ensure it contains at least one non-empty pattern
2. Skip the multiGrep call entirely (or return an empty result) when the pattern list is empty
3. Add a fallback default pattern when the caller's list filters down to zero

Example fix

// before
finder.multiGrep({ patterns: myPatterns, constraints: '' });
// after
if (!myPatterns.length) return;
finder.multiGrep({ patterns: myPatterns.length ? myPatterns : ['*'], constraints: '' });

if (!Array.isArray(patterns) || patterns.length === 0) throw new Error('multiGrep needs at least 1 pattern');

function hasPatterns(o) { return Array.isArray(o?.patterns) && o.patterns.length > 0; }

try { finder.multiGrep(opts); } catch (e) { if (String(e).includes('patterns array')) { /* recover: use fallback pattern or skip */ } }

When it happens

Trigger: Calling finder.multiGrep() with no options.patterns, with patterns: [], or with patterns left undefined after building options dynamically.

Common situations: Building the pattern list programmatically from user input or config where filtering removed all patterns; forgetting to default to a fallback pattern list.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• fff native library not found. Run `npx @ff-labs/fff-node…
• Query is null or invalid UTF-8
• File path is null or invalid UTF-8
• Failed to canonicalize path
• Unsupported FffWatchOptions version

AI-assisted analysis of dmtrKovalenko/fff@7f8537e70f (2026-09-10). Data as JSON: /api/errors/b49b53c1cdf33f99. Report an issue: GitHub.

Appendix: source

Thrown at packages/fff-bun/src/finder.ts:413

   *
   * @example
   * ```typescript
   * const result = finder.multiGrep({
   *   patterns: ["VideoFrame", "video_frame", "PreloadedImage"],
   * });
   * if (result.ok) {
   *   for (const match of result.value.items) {
   *     console.log(`${match.relativePath}:${match.lineNumber}: ${match.lineContent}`);
   *   }
   * }
   * ```
   */
  multiGrep(options: MultiGrepOptions): Result<GrepResult> {
    const guard = this.ensureAlive();
    if (!guard.ok) return guard;

    if (!options.patterns || options.patterns.length === 0) {
      return err("patterns array must have at least 1 element");
    }

    return ffiMultiGrep(
      guard.value,
      options.patterns.join("\n"),
      options.constraints ?? "",
      options.maxFileSize ?? 0,
      options.maxMatchesPerFile ?? 0,
      options.smartCase ?? true,
      options.cursor?._offset ?? 0,
      options.pageSize ?? 0,
      options.timeBudgetMs ?? 0,
      options.enforceTimeBudget ?? false,
      options.beforeContext ?? 0,
      options.afterContext ?? 0,
      options.classifyDefinitions ?? false,
    );
  }

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