mozilla/pdf.js · error · TypeError

Invalid field name: must be a string

Error message

Invalid field name: must be a string

What it means

Thrown inside `Doc._getField(cName)` (src/scripting_api/doc.js:931), the helper behind `doc.getField()` and several field-lookup APIs. After unwrapping an optional `{cName}` object, it asserts the remaining value is a string. It is a `TypeError`, reflecting that the field-name parameter has the wrong type, not just a missing field.

Source

Thrown at src/scripting_api/doc.js:931

  getColorConvertAction() {
    /* Not implemented */
  }

  getDataObject() {
    /* Not implemented */
  }

  getDataObjectContents() {
    /* Not implemented */
  }

  _getField(cName) {
    if (cName && typeof cName === "object") {
      cName = cName.cName;
    }
    if (typeof cName !== "string") {
      throw new TypeError("Invalid field name: must be a string");
    }
    const searchedField = this._fields.get(cName);
    if (searchedField) {
      return searchedField;
    }

    const parts = cName.split("#");
    let childIndex = NaN;
    if (parts.length === 2) {
      childIndex = Math.floor(parseFloat(parts[1]));
      cName = parts[0];
    }

    for (const [name, field] of this._fields) {
      if (name.endsWith(cName)) {
        if (!isNaN(childIndex)) {
          const children = this._getChildren(name);
          if (childIndex < 0 || childIndex >= children.length) {

View on GitHub (pinned to 5903d58d58)

Solutions

1. Coerce the lookup key to a string before calling: `doc.getField(String(fieldName))`.
2. Guard against undefined/null and return early when the name is absent.
3. When passing an object, ensure the property key is exactly `cName` (e.g. `{cName: 'foo'}`).

Example fix

// before
const f = doc.getField(this.getFieldAt(i)); // index passed by mistake
// after
const name = doc.getNthFieldName(i);
const f = name ? doc.getField(String(name)) : null;

function getFieldSafe(doc, cName) {
  if (cName && typeof cName === 'object') cName = cName.cName;
  if (typeof cName !== 'string' || cName.length === 0) return null;
  return doc.getField(cName);
}

function isFieldName(v) {
  if (v && typeof v === 'object') v = v.cName;
  return typeof v === 'string' && v.length > 0;
}

When it happens

Trigger: Calling `doc.getField()` with no argument, or with a number/boolean/Array, e.g. `doc.getField(3)`, `doc.getField()`, `doc.getField(someNumber)`. The object-unwrap (`cName = cName.cName`) only fires when the argument is an object, so passing `{nIndex: 0}` (wrong key) falls through and throws.

Common situations: Dynamic field lookups where the name comes from a variable that is sometimes a number (an index) or undefined; destructured parameter objects whose key does not match `cName`; calling `getField` from a loop counter without `.toString()`.

Related errors

• Invalid field index: must be a number
• Invalid argument value
• First argument of printf must be a string
• app.activeDocs is read-only
• app.constants is read-only

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/d582eecce5e3bb73. Report an issue: GitHub.