What causes "doc.URL is read-only" in mozilla/pdf.js?

Remove the assignment; read the value (`var u = doc.URL;`).

mozilla/pdf.js · error · Error

doc.URL is read-only

Error message

doc.URL is read-only

What it means

`Doc.URL` returns the URL from which the document was loaded (`this._URL`). It is read-only per the Acrobat spec — the source URL is a factual attribute of how the doc was opened, not settable by in-document scripts. PDF.js's throwing setter enforces that contract.

Source

Thrown at src/scripting_api/doc.js:687

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

  get title() {
    return this._title;
  }

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

  get URL() {
    return this._URL;
  }

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

  get viewState() {
    return undefined;
  }

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

  get xfa() {
    return this._xfa;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

Remove the assignment; read the value (`var u = doc.URL;`).
Control the source URL from the embedding page / `getDocument` call rather than from inside a document script.
Wrap script execution in try/catch.

Example fix

// before
doc.URL = 'https://example.com/doc.pdf';

// after
var u = doc.URL; // read-only source URL

Defensive patterns

Strategy: validation

Validate before calling

const READ_ONLY_DOC_PROPS = new Set(['URL','path' /* ... */]);
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, 'URL'); // true

Try / catch

try {
  doc.URL = 'https://example.com/doc.pdf';
} catch (e) {
  if (/\\bURL is read-only/.test(e.message)) console.warn(e.message);
  else throw e;
}

Prevention

Source URL/path is set by how the doc is loaded; control it from `getDocument`/the embedding page.
Read `doc.URL` rather than writing it.
Lint scripts for assignments to read-only source properties.

When it happens

Trigger: A script writes `doc.URL = 'https://...';` to redirect or record the source. The setter throws on that line.

Common situations: Scripts that try to force a reload from a new URL; branding/audit code that rewrites the URL field; legacy scripts ported from a local-file context.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/2cd1d638edd57a88. Report an issue: GitHub.