mozilla/pdf.js · error · Error

app.thermometer is read-only

Error message

app.thermometer is read-only

What it means

app.thermometer is a lazily-created Proxy around a Thermometer progress helper that communicates via the send callback. It is viewer-internal state, so the setter throws. Reassigning would detach the progress UI wiring.

Source

Thrown at src/scripting_api/app.js:353

    return this._runtimeHighlightColor;
  }

  set runtimeHighlightColor(val) {
    if (Color._isValidColor(val)) {
      this._runtimeHighlightColor = val;
      /* TODO */
    }
  }

  get thermometer() {
    return (this._thermometer ??= new Proxy(
      new Thermometer({ send: this._send }),
      this._proxyHandler
    ));
  }

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

  get toolbar() {
    return this._toolbar;
  }

  set toolbar(val) {
    this._toolbar = val;
    /* TODO */
  }

  get toolbarHorizontal() {
    return this.toolbar;
  }

  set toolbarHorizontal(value) {
    /* has been deprecated and it's now equivalent to toolbar */
    this.toolbar = value;

View on GitHub (pinned to 5903d58d58)

Solutions

1. Use the returned Thermometer proxy's API (begin/update/end) instead of reassigning app.thermometer.
2. For tests, mock at the underlying _externalCall/send layer.
3. Read app.thermometer to obtain the current progress object.

Example fix

// before
app.thermometer = myProgress;
// after
const t = app.thermometer; t.begin(); t.update(50);

// app.thermometer is a viewer-managed progress proxy; use begin/update/end.
function runWithProgress(app, fn) {
  const t = app.thermometer;
  if (!t || typeof t.begin !== 'function') throw new Error('Thermometer unavailable');
  t.begin(); try { return fn(t); } finally { t.end(); }
}

const isThermometer = (v) =>
  v != null && typeof v === 'object' && typeof v.begin === 'function';

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

When it happens

Trigger: Any assignment app.thermometer = ... in PDF JavaScript or API test code hits the throw at src/scripting_api/app.js:353.

Common situations: Long-running scripts trying to install a custom progress object; tests attempting to mock progress by overwriting the property.

Related errors

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

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/4e8011b17fea391d. Report an issue: GitHub.