mozilla/pdf.js · error · Error

doc.numFields is read-only

Error message

doc.numFields is read-only

What it means

`Doc.numFields` returns the count of interactive fields in the document, backed by `this._numFields` (derived from the field map built when the document is parsed). Acrobat defines it as read-only because field count only changes through `addField`/`removeField`, not by direct assignment. PDF.js's throwing setter enforces that contract so scripts get an immediate, spec-consistent failure instead of a silently-ignored write.

Source

Thrown at src/scripting_api/doc.js:526

  set noautocomplete(noautocomplete) {
    this._noautocomplete = noautocomplete;
  }

  get nocache() {
    return this._nocache;
  }

  set nocache(nocache) {
    this._nocache = nocache;
  }

  get numFields() {
    return this._numFields;
  }

  set numFields(_) {
    throw new Error("doc.numFields is read-only");
  }

  get numPages() {
    return this._numPages;
  }

  set numPages(_) {
    throw new Error("doc.numPages is read-only");
  }

  get numTemplates() {
    return 0;
  }

  set numTemplates(_) {
    throw new Error("doc.numTemplates is read-only");
  }

View on GitHub (pinned to 5903d58d58)

Solutions

  1. Use `addField(...)` / `removeField(...)` to change the count, then read `doc.numFields` to observe it.
  2. Replace any `doc.numFields = ...` with a local variable that tracks your own counter.
  3. Gate legacy scripts with a try/catch at the host so the error is non-fatal.

Example fix

// before
doc.numFields = doc.numFields + 1;

// after
doc.addField('NewField', 'text', [0], 0);
console.log(doc.numFields); // now reflects the new count
Defensive patterns

Strategy: validation

Validate before calling

const READ_ONLY_DOC_PROPS = new Set(['numFields','numPages' /* ... */]);
function assertsNoReadOnlyWrite(scriptSrc) {
  for (const p of READ_ONLY_DOC_PROPS) {
    if (new RegExp(`\\bdoc\\.${p}\\s*=[^=]`).test(scriptSrc)) {
      throw new Error(`Script writes read-only property doc.${p}`);
    }
  }
}
assertsNoReadOnlyWrite(myScript);

Type guard

function isReadOnlyDocProp(doc, prop) {
  const desc = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(doc), prop)
    || Object.getOwnPropertyDescriptor(doc, prop);
  return !!desc && !!desc.get && !desc.set;
}
isReadOnlyDocProp(doc, 'numFields'); // true

Try / catch

try {
  doc.numFields = n;
} catch (e) {
  if (/numFields is read-only/.test(e.message)) console.warn(e.message);
  else throw e;
}

Prevention

When it happens

Trigger: A script writes `doc.numFields = n;` or `doc.numFields++`, often as a misguided attempt to pre-size or reset the field collection. The throwing setter aborts the action.

Common situations: A script incorrectly assumes `numFields` is a mutable capacity/length; code ported from a different form engine where field count is settable; debugging code that assigns to counters.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/60dfbebe7f8fca04. Report an issue: GitHub.