mozilla/pdf.js · error · Error

doc.viewState is read-only

Error message

doc.viewState is read-only

What it means

`Doc.viewState` returns the document's view state (used to capture/restore the viewing position); PDF.js returns `undefined` because it does not expose a serializable view-state object to the scripting sandbox. It is read-only per the Acrobat spec — view state is observed, not assigned through this property (restoration uses dedicated APIs). The throwing setter enforces that contract.

Source

Thrown at src/scripting_api/doc.js:695

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

  get URL() {
    return this._URL;
  }

  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");
  }

View on GitHub (pinned to 5903d58d58)

Solutions

  1. Remove the assignment; read the value (`var v = doc.viewState;`, `undefined` in PDF.js).
  2. Implement view capture/restore at the host viewer layer (scroll/zoom/page state) instead of via this property.
  3. Run legacy scripts through try/catch.

Example fix

// before
doc.viewState = { page: 1, zoom: 1.5 };

// after
var v = doc.viewState; // read-only; undefined in PDF.js
Defensive patterns

Strategy: try-catch

Validate before calling

const READ_ONLY_DOC_PROPS = new Set(['viewState','pageWindowRect' /* ... */]);
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, 'viewState'); // true

Try / catch

try {
  doc.viewState = { page: 1, zoom: 1.5 };
} catch (e) {
  if (/viewState is read-only/.test(e.message)) console.warn(e.message);
  else throw e;
}

Prevention

When it happens

Trigger: A script writes `doc.viewState = {...};`, attempting to capture or force a view state. The setter throws on assignment.

Common situations: Layout-restore scripts ported from Acrobat; code that snapshots view state on close; tutorials that demonstrate unsupported writes.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/a412ebe2f67d6c90. Report an issue: GitHub.