mozilla/pdf.js · error · Error

app.viewerType is read-only

Error message

app.viewerType is read-only

What it means

app.viewerType exposes the compiled VIEWER_TYPE constant (identifying PDF.js as the viewer). It is fixed per build and read-only; the setter throws. The throw prevents spoofing the viewer identity.

Source

Thrown at src/scripting_api/app.js:388

    /* has been deprecated and it's now equivalent to toolbar */
    this.toolbar = value;
  }

  get toolbarVertical() {
    return this.toolbar;
  }

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

  get viewerType() {
    return VIEWER_TYPE;
  }

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

  get viewerVariation() {
    return VIEWER_VARIATION;
  }

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

  get viewerVersion() {
    return VIEWER_VERSION;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

1. Read app.viewerType for viewer-specific branching (expect the PDF.js identifier).
2. Gate features on your own capability checks, not by rewriting viewerType.
3. If a different identifier is truly required, build PDF.js from source with a custom constant.

Example fix

// before
app.viewerType = 'Exchange-Pro';
// after
if (app.viewerType === VIEWER_TYPE) { /* pdf.js path */ }

// app.viewerType is a build constant; read only.
const isPdfJs = (app) => app.viewerType === VIEWER_TYPE;

const isViewerTypeString = (v) => typeof v === 'string' && v.length > 0;

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

When it happens

Trigger: Any assignment app.viewerType = 'Exchange-Pro' (or any value) in PDF JavaScript or caller code hits the throw at src/scripting_api/app.js:388.

Common situations: Scripts trying to impersonate Acrobat Professional to unlock menu actions; feature checks that mistakenly write instead of read.

Related errors

• app.formsVersion is read-only
• app.viewerVariation is read-only
• app.viewerVersion 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/93974b3fa243bd97. Report an issue: GitHub.