mozilla/pdf.js · error · Error

app.numPlugins is read-only

Error message

app.numPlugins is read-only

What it means

app.numPlugins returns 0 because PDF.js loads no Acrobat-style plugins. It is a viewer-managed count and is not assignable; the setter throws. The throw prevents spoofing the plugin count.

Source

Thrown at src/scripting_api/app.js:281

  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 */
  }

  get platform() {
    return this._platform;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

1. Read app.numPlugins (always 0 in PDF.js) for capability detection.
2. Gate plugin-dependent features on app.numPlugins > 0, expecting false.
3. Do not attempt to change the value from document scripts.

Example fix

// before
app.numPlugins = 3;
// after
if (app.numPlugins > 0) { /* ... */ } else { /* unsupported */ }

// app.numPlugins is always 0 in pdf.js.
const pluginsAvailable = (app) => app.numPlugins > 0;

const isNonNegativeInt = (v) => Number.isInteger(v) && v >= 0;

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

When it happens

Trigger: PDF JavaScript or caller runs app.numPlugins = N; the setter at src/scripting_api/app.js:281 throws regardless of value.

Common situations: Scripts trying to fake plugin availability to gate features; authors confusing the stub with a registry length.

Related errors

• app.fromPDFConverters is read-only
• app.monitors 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/af10e673dcebe224. Report an issue: GitHub.