mozilla/pdf.js · error · Error

app.monitors is read-only

Error message

app.monitors is read-only

What it means

app.monitors returns an empty array (PDF.js exposes no monitor enumeration). It is a viewer-managed list, so the setter throws. Assignment is rejected unconditionally.

Source

Thrown at src/scripting_api/app.js:273

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

  get media() {
    return undefined;
  }

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

  get monitors() {
    return [];
  }

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

  get numPlugins() {
    return 0;
  }

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

  get openInPlace() {
    return this._openInPlace;
  }

  set openInPlace(val) {
    this._openInPlace = val;
    /* TODO */
  }

View on GitHub (pinned to 5903d58d58)

Solutions

1. Read app.monitors only; expect [] in PDF.js.
2. Obtain screen geometry from the host page/window APIs instead.
3. Do not attempt to push into the returned array expecting persistence.

Example fix

// before
app.monitors = [{ width: 1920 }];
// after
const monitors = app.monitors; // [] in pdf.js; use host APIs instead

// app.monitors always returns [] in pdf.js.
const monitorCount = (app) => app.monitors.length;

const isMonitorList = (v) => Array.isArray(v);

try { /* code that may assign app.monitors */ }
catch (e) { if (!/monitors is read-only/.test(e.message)) throw e; }

When it happens

Trigger: Any assignment app.monitors = [...] in PDF JavaScript or API test code hits the throw at src/scripting_api/app.js:273.

Common situations: Scripts trying to register or fake monitor entries for layout logic; authors assuming the array is extensible.

Related errors

• app.fromPDFConverters is read-only
• app.numPlugins is read-only
• app.plugins is read-only
• app.activeDocs is read-only
• app.constants is read-only

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/d89aaad698612247. Report an issue: GitHub.