mozilla/pdf.js · error · Error

doc.sounds is read-only

Error message

doc.sounds is read-only

What it means

`Doc.sounds` returns the array of sound objects embedded in the document; PDF.js returns `[]` because sound assets are not surfaced to the scripting sandbox. It is read-only per the Acrobat spec — the sound collection is part of document content, not settable. The throwing setter enforces that contract.

Source

Thrown at src/scripting_api/doc.js:639

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

  get selectedAnnots() {
    return [];
  }

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

  get sounds() {
    return [];
  }

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

  get spellDictionaryOrder() {
    return this._spellDictionaryOrder;
  }

  set spellDictionaryOrder(spellDictionaryOrder) {
    this._spellDictionaryOrder = spellDictionaryOrder;
  }

  get spellLanguageOrder() {
    return this._spellLanguageOrder;
  }

  set spellLanguageOrder(spellLanguageOrder) {
    this._spellLanguageOrder = spellLanguageOrder;
  }

View on GitHub (pinned to 5903d58d58)

Solutions

  1. Remove the assignment; read the value (`var s = doc.sounds;`, `[]` in PDF.js).
  2. Handle audio at the host integration layer if needed.
  3. Guard legacy scripts with try/catch.

Example fix

// before
doc.sounds = [mySound];

// after
var s = doc.sounds; // read-only; [] in PDF.js
Defensive patterns

Strategy: try-catch

Validate before calling

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

Try / catch

try {
  doc.sounds = [sound];
} catch (e) {
  if (/sounds is read-only/.test(e.message)) console.warn(e.message);
  else throw e;
}

Prevention

When it happens

Trigger: A script writes `doc.sounds = [...];` to register or replace sounds. The setter throws immediately.

Common situations: Media-rich forms ported from Acrobat; code that plays sound on events and tries to manage the sound list; legacy interactive PDFs.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/2afa6e2d3a3f9e2f. Report an issue: GitHub.