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

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

mozilla/pdf.js · error · Error

doc.isModal is read-only

Error message

doc.isModal 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. `isModal` indicates whether the document is in a modal state; pdf.js always returns `false` and forbids writes.

Source

Thrown at src/scripting_api/doc.js:426

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

  get innerDocWindowRect() {
    return [0, 0, 0, 0];
  }

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

  get isModal() {
    return false;
  }

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

  get keywords() {
    return this._keywords;
  }

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

  get layout() {
    return this._layout;
  }

  set layout(value) {
    if (!this._userActivation) {
      return;
    }

View on GitHub (pinned to 5903d58d58)

Solutions

Remove the assignment to `doc.isModal`; read it through the getter instead.
pdf.js has no modal document mode to enter; remove the assignment.
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.isModal = true;
// after
// isModal is read-only — drop the assignment; read via doc.isModal

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('isModal')) {
  // skip the write; isModal is read-only in pdf.js
  console.warn('doc.isModal is read-only in pdf.js');
} else {
  doc.isModal = 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.isModal = value;
} catch (e) {
  // pdf.js throws Error('doc.isModal 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('isModal')` 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.isModal = value;` or `doc.isModal++`. Because the setter always throws (there is no condition), any write attempt triggers it.

Common situations: Scripts ported from desktop Acrobat where modality was 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/68aa74f4f8edbc15. Report an issue: GitHub.