mozilla/pdf.js · error · Error

doc.xfa is read-only

Error message

doc.xfa is read-only

What it means

Thrown by the setter of the `xfa` property on the Acrobat JavaScript `Doc` object in PDF.js's scripting sandbox (src/scripting_api/doc.js:702). In the Acrobat API, `doc.xfa` exposes the underlying XFA (XML Forms Architecture) DOM node; PDF.js treats it as an intrinsic, non-writable attribute and throws on any assignment. The getter returns `this._xfa` (the XFA object parsed by the core layer, or undefined for non-XFA documents).

Source

Thrown at src/scripting_api/doc.js:703

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

  get viewState() {
    return undefined;
  }

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

  get xfa() {
    return this._xfa;
  }

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

  get XFAForeground() {
    return false;
  }

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

  get zoomType() {
    return this._zoomType;
  }

  set zoomType(type) {
    if (!this._userActivation) {
      return;
    }

View on GitHub (pinned to 5903d58d58)

Solutions

1. Remove the assignment to `doc.xfa` — it is read-only by design; mutate the XFA tree through its own methods instead of replacing the reference.
2. If a generic routine is assigning, skip read-only members by name before writing (guard with a known-readonly list).
3. Wrap the assignment in try/catch if the write is best-effort, so a single read-only field does not abort the whole script.

Example fix

// before
this.doc.xfa = newXfa;
// after
// do not reassign; xfa is read-only. Read it only:
const xfa = this.doc.xfa;

// These Doc properties are intrinsic and never writable in PDF.js.
const READ_ONLY_DOC_PROPS = new Set(['URL','viewState','xfa','XFAForeground']);
function safeAssignDoc(doc, key, value) {
  if (READ_ONLY_DOC_PROPS.has(key)) return false; // skip
  doc[key] = value;
  return true;
}

try {
  doc.xfa = candidate;
} catch (e) {
  // xfa is read-only in PDF.js; proceed without the assignment
}

When it happens

Trigger: A script embedded in a PDF assigns to `doc.xfa` (or `this.doc.xfa`), e.g. `this.doc.xfa = newXfa;`, or generic form-filler code that copies whole objects via `Object.assign(doc, {...})` which hits the setter. Also triggered by libraries that try to 'reset' a form by reassigning every enumerable-looking property.

Common situations: XFA form migration where an author assumed the XFA tree could be swapped at runtime; porting an Acrobat script from a reader that silently ignored the assignment; an aggressive form-reset routine that blindly writes known property names.

Related errors

• doc.XFAForeground is read-only
• app.activeDocs is read-only
• app.constants is read-only
• app.formsVersion is read-only
• app.fromPDFConverters is read-only

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/a8bbdcb8177bc7b8. Report an issue: GitHub.