mozilla/pdf.js · error · Error

app.constants is read-only

Error message

app.constants is read-only

What it means

app.constants is a lazily-built, Object.freeze'd namespace (currently exposing align.left/center/right/top/bottom). Because it is a fixed enumeration surface in the Acrobat JS API, PDF.js freezes it and makes the setter throw. Any assignment to app.constants is rejected.

Source

Thrown at src/scripting_api/app.js:213

  set calculate(calculate) {
    this._document.obj.calculate = calculate;
  }

  get constants() {
    return (this._constants ??= Object.freeze({
      align: Object.freeze({
        left: 0,
        center: 1,
        right: 2,
        top: 3,
        bottom: 4,
      }),
    }));
  }

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

  get focusRect() {
    return this._focusRect;
  }

  set focusRect(val) {
    /* TODO or not */
    this._focusRect = val;
  }

  get formsVersion() {
    return FORMS_VERSION;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

1. Do not assign to app.constants; treat it as a frozen enumeration.
2. Define your own module-level constants object for any custom values.
3. Read app.constants.align.<name> only for the supported keys (left, center, right, top, bottom).

Example fix

// before
app.constants = { align: { left: 0 } };
// after
const myAlign = { left: 0 }; // keep custom constants separate

// app.constants is frozen and read-only.
function safeConstant(app, group, key) {
  const c = app.constants;
  if (!c || !Object.isFrozen(c)) throw new Error('Unexpected constants shape');
  return c?.[group]?.[key];
}

const isReadOnlyAppProp = (prop) =>
  Object.getOwnPropertyDescriptor(App.prototype, prop)?.set?.name === '';
// (setter is an arrow-less throw stub; alternatively maintain an explicit list)

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

When it happens

Trigger: PDF JavaScript or test code runs app.constants = {...} or attempts app.constants.align = .... Reassigning the property triggers the setter throw at src/scripting_api/app.js:213; mutating inner frozen objects throws a separate TypeError from Object.freeze.

Common situations: Scripts that try to extend the constants enumeration with custom alignment values; authors confusing app.constants with a user-extensible config object.

Related errors

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

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/a0ec352e07201958. Report an issue: GitHub.