mozilla/pdf.js · error · Error

app.platform is read-only

Error message

app.platform is read-only

What it means

app.platform returns the detected platform string (this._platform). It reflects the host environment and is not user-writable, so the setter throws. The throw preserves the platform detection source-of-truth.

Source

Thrown at src/scripting_api/app.js:298

  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");
  }

  get plugins() {
    return [];
  }

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

  get printColorProfiles() {
    return [];
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

1. Read app.platform for platform-specific document behavior.
2. If you need a different platform value in tests, construct the App with the desired _platform rather than assigning the property.
3. Do not attempt to mutate app.platform from document JavaScript.

Example fix

// before
app.platform = 'MAC';
// after
if (app.platform === 'MAC') { /* mac code path */ }

// app.platform is detected at runtime; read only.
const isMac = (app) => app.platform === 'MAC';

const isKnownPlatform = (v) => ['WIN','MAC','UNIX'].includes(v);

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

When it happens

Trigger: PDF-embedded JavaScript or caller executes app.platform = 'Win' (or any value); the setter at src/scripting_api/app.js:298 throws unconditionally.

Common situations: Cross-platform scripts trying to spoof platform to force a code path; tests that write instead of mocking the constructor.

Related errors

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

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/dc4f969520843b67. Report an issue: GitHub.