mozilla/pdf.js · error · Error

doc.numTemplates is read-only

Error message

doc.numTemplates is read-only

What it means

`Doc.numTemplates` returns the number of page templates in the document; PDF.js always returns 0 because the page-template feature is not implemented. It is read-only per the Acrobat spec — templates are managed via `createTemplate`/`removeTemplate`, not by setting a count. The throwing setter keeps PDF.js behavior aligned with Acrobat's contract.

Source

Thrown at src/scripting_api/doc.js:542

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

  get numPages() {
    return this._numPages;
  }

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

  get numTemplates() {
    return 0;
  }

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

  get outerAppWindowRect() {
    return [0, 0, 0, 0];
  }

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

  get outerDocWindowRect() {
    return [0, 0, 0, 0];
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

  1. Remove the assignment; `numTemplates` is observable only (`var n = doc.numTemplates;`, always 0 in PDF.js).
  2. If template behavior is required, implement it at the host/integration layer rather than via this property.
  3. Guard execution of legacy scripts with try/catch to tolerate the unsupported write.

Example fix

// before
doc.numTemplates = 3;

// after
var n = doc.numTemplates; // 0 in PDF.js; templates unsupported
Defensive patterns

Strategy: try-catch

Validate before calling

const READ_ONLY_DOC_PROPS = new Set(['numTemplates','templates' /* ... */]);
function assertsNoReadOnlyWrite(scriptSrc) {
  for (const p of READ_ONLY_DOC_PROPS) {
    if (new RegExp(`\\bdoc\\.${p}\\s*=[^=]`).test(scriptSrc)) {
      throw new Error(`Script writes read-only property doc.${p}`);
    }
  }
}
assertsNoReadOnlyWrite(myScript);

Type guard

function isReadOnlyDocProp(doc, prop) {
  const desc = Object.getOwnPropertyDescriptor(Object.getPrototypeOf(doc), prop)
    || Object.getOwnPropertyDescriptor(doc, prop);
  return !!desc && !!desc.get && !desc.set;
}
isReadOnlyDocProp(doc, 'numTemplates'); // true

Try / catch

try {
  doc.numTemplates = 3;
} catch (e) {
  if (/numTemplates is read-only/.test(e.message)) console.warn(e.message);
  else throw e;
}

Prevention

When it happens

Trigger: A script writes `doc.numTemplates = n;` or increments it. Because templates are unsupported, the value would be meaningless to set even if the setter accepted it; instead it throws.

Common situations: Acrobat SDK sample code that demonstrates template creation; legacy dynamic-form scripts; scripts unaware that PDF.js does not implement page templates.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/59091bbed8089949. Report an issue: GitHub.