mozilla/pdf.js · error · Error

app.language is read-only

Error message

app.language is read-only

What it means

app.language returns the viewer's resolved language string (this._language). It reflects the host locale and is not user-writable, so the setter throws. The throw guards the locale source-of-truth held by the viewer.

Source

Thrown at src/scripting_api/app.js:257

  }

  get fs() {
    return (this._fs ??= new Proxy(
      new FullScreen({ send: this._send }),
      this._proxyHandler
    ));
  }

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

  get language() {
    return this._language;
  }

  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");
  }

View on GitHub (pinned to 5903d58d58)

Solutions

1. Set the desired locale at the viewer/host level (e.g., PDF.js locale init), not via app.language.
2. Read app.language to branch document behavior per locale.
3. Pass language preferences through viewer configuration before document load.

Example fix

// before
app.language = 'FRA';
// after
// configure pdf.js with the fr locale at startup; then:
if (app.language === 'FRA') { /* ... */ }

// app.language reflects host locale; configure via viewer init.
function pickLabel(app, dict) { return dict[app.language] ?? dict.default; }

const isLanguageString = (v) => typeof v === 'string' && /^[A-Z]{3}$/.test(v);

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

When it happens

Trigger: PDF-embedded JavaScript or caller executes app.language = 'FRA' (or any value); the setter at src/scripting_api/app.js:257 throws unconditionally.

Common situations: Scripts trying to force a UI language from within the document; localization tests that mistakenly write instead of configuring the viewer.

Related errors

• app.activeDocs is read-only
• app.constants is read-only
• app.formsVersion is read-only
• app.fromPDFConverters is read-only
• app.monitors is read-only

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/2e57ddb60555e206. Report an issue: GitHub.