overleaf/overleaf · error · Error

FileData: toStats not implemented

Error message

FileData: toStats not implemented

What it means

toStats() returns a Record<string, number> of file statistics (e.g. byte/string lengths) and is another abstract stub on the FileData base class. Every concrete subclass must override it; hitting the stub means the receiver has no stats implementation. Throwing keeps stats consumers from reading meaningless defaults like 0.

Source

Thrown at libraries/overleaf-editor-core/lib/file_data/index.js:88

    return new LazyStringFileData(
      blob.getHash(),
      rangesBlob?.getHash(),
      stringLength
    )
  }

  /**
   * @returns {RawFileData}
   */
  toRaw() {
    throw new Error('FileData: toRaw not implemented')
  }

  /**
   * @returns {Record<string, number>}
   */
  toStats() {
    throw new Error('FileData: toStats not implemented')
  }

  /**
   * @see File#getHash
   * @return {string | null | undefined}
   */

  getHash() {
    return null
  }

  /**
   * @see File#getHash
   * @return {string | null | undefined}
   */
  getRangesHash() {
    return null
  }

View on GitHub (pinned to 28ad3b03b7)

Solutions

  1. Add a toStats() override to the offending subclass returning its numeric stats (e.g. {byteLength, stringLength}).
  2. Replace ad-hoc subclass instances with the library's concrete classes that already implement toStats.
  3. Update subclasses after upgrading overleaf-editor-core to satisfy any newly added abstract members.

Example fix

// before
class CustomFileData extends FileData {}
// after
class CustomFileData extends FileData {
  toStats() {
    return { byteLength: this.getByteLength() || 0 }
  }
}
Defensive patterns

Strategy: type-guard

Validate before calling

if (FileData.prototype.toStats === fileData.toStats) {
  throw new Error(fileData.constructor.name + ' does not implement toStats')
}

Type guard

const implementsToStats = (fd) =>
  fd instanceof FileData && FileData.prototype.toStats !== fd.toStats

Try / catch

let stats
try {
  stats = fileData.toStats()
} catch (err) {
  if (err.message === 'FileData: toStats not implemented') {
    stats = {}
  } else {
    throw err
  }
}

Prevention

When it happens

Trigger: Calling toStats() on a bare new FileData() or on a custom subclass that overrides toRaw/edit but not toStats; invoking it through File wrapper methods on such an instance.

Common situations: Custom or test-double FileData subclasses; version drift where a new abstract method (toStats) was added and older subclasses in your codebase were not updated; directly instantiating the base class.

Understand the failure class

Background: "NotImplementedError: Subclasses should override this method" / "must be implemented" — abstract method errors explained — this error's family across 40 libraries.

Related errors

AI-assisted analysis of overleaf/overleaf@28ad3b03b7 (2026-09-03). Data as JSON: /api/errors/f53a014b13113adb. Report an issue: GitHub.