What causes "doc.dynamicXFAForm is read-only" in mozilla/pdf.js?

Remove the assignment to `doc.dynamicXFAForm`; read it through the getter instead.

mozilla/pdf.js · error · Error

doc.dynamicXFAForm is read-only

Error message

doc.dynamicXFAForm is read-only

What it means

PDF.js's scripting sandbox (src/scripting_api) implements the Acrobat JavaScript `doc` object inside a QuickJS sandbox. Per the Acrobat API specification this property is read-only, so the class defines a getter that returns the current value and a setter that unconditionally throws `Error("doc.<prop> is read-only")`. The throw aborts the currently executing sandboxed script event and is reported back to the host. `dynamicXFAForm` indicates whether the form is a dynamic XFA form; pdf.js always returns `false` (static/no XFA) and forbids writes.

Source

Thrown at src/scripting_api/doc.js:351

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

  get documentFileName() {
    return this._documentFileName;
  }

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

  get dynamicXFAForm() {
    return false;
  }

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

  get external() {
    // According to the specification this should be `true` in non-Acrobat
    // applications, however we ignore that to avoid bothering users with
    // an `alert`-dialog on document load (see issue 15509).
    return DOC_EXTERNAL;
  }

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

  get filesize() {
    return this._filesize;
  }

  set filesize(_) {

View on GitHub (pinned to 5903d58d58)

Solutions

Remove the assignment to `doc.dynamicXFAForm`; read it through the getter instead.
pdf.js does not support dynamic XFA; convert the form to AcroForm or accept static behavior.
If you cannot edit the PDF's embedded script, wrap the statement in try/catch so the rest of the form logic still runs.

Example fix

// before
doc.dynamicXFAForm = true;
// after
// dynamicXFAForm is read-only — drop the assignment; read via doc.dynamicXFAForm

Defensive patterns

Strategy: validation

Validate before calling

// Known read-only doc properties (PDF.js scripting_api/doc.js)
const READONLY_DOC_PROPS = new Set([
  'author','bookmarkRoot','creator','dataObjects','docID',
  'documentFileName','dynamicXFAForm','external','filesize','hidden',
  'hostContainer','icons','info','innerAppWindowRect','innerDocWindowRect',
  'isModal','keywords','modDate'
]);
if (READONLY_DOC_PROPS.has('dynamicXFAForm')) {
  // skip the write; dynamicXFAForm is read-only in pdf.js
  console.warn('doc.dynamicXFAForm is read-only in pdf.js');
} else {
  doc.dynamicXFAForm = value;
}

Type guard

// Known read-only doc properties (PDF.js scripting_api/doc.js)
const READONLY_DOC_PROPS = new Set([
  'author','bookmarkRoot','creator','dataObjects','docID',
  'documentFileName','dynamicXFAForm','external','filesize','hidden',
  'hostContainer','icons','info','innerAppWindowRect','innerDocWindowRect',
  'isModal','keywords','modDate'
]);
const isReadOnlyDocProp = (name) => READONLY_DOC_PROPS.has(name);
// usage: if (!isReadOnlyDocProp('keywords')) doc.keywords = v;

Try / catch

try {
  doc.dynamicXFAForm = value;
} catch (e) {
  // pdf.js throws Error('doc.dynamicXFAForm is read-only')
  if (/is read-only/.test(e.message)) { /* swallow, expected */ }
  else { throw e; }
}

Prevention

Treat every property listed in the Acrobat JS reference as read-only unless pdf.js provides a setter that stores the value.
Before assigning, check `isReadOnlyDocProp('dynamicXFAForm')` or grep the setter in src/scripting_api/doc.js for a `throw`.
When porting scripts from Acrobat, run them against the pdf.js sandbox in the dev server first to surface writes early.
Prefer reading metadata through the Info dictionary at PDF authoring time rather than mutating it from a script.

When it happens

Trigger: A sandboxed PDF form/script executes an assignment to the property, e.g. `doc.dynamicXFAForm = value;` or `doc.dynamicXFAForm++`. Because the setter always throws (there is no condition), any write attempt triggers it.

Common situations: Scripts ported from desktop Acrobat where XFA flags were toggled; forms that try to stamp runtime state into document metadata on save/open; and PDFs generated by tools that emit non-spec-compliant JavaScript.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/86081afde6f34443. Report an issue: GitHub.