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

Remove the assignment to `doc.modDate`; read it through the getter instead.

mozilla/pdf.js · error · Error

doc.modDate is read-only

Error message

doc.modDate is read-only

What it means

PDF.js's scripting sandbox (src/scripting_api) implements the Acrobat JavaScript `doc` object inside a QuickJS sandbox. Per the Acrobat API specification this property is read-only, so the class defines a getter that returns the current value and a setter that unconditionally throws `Error("doc.<prop> is read-only")`. The throw aborts the currently executing sandboxed script event and is reported back to the host. `modDate` is the document last-modification Date (seeded from `data.ModDate`); read-only per spec.

Source

Thrown at src/scripting_api/doc.js:486

  set media(media) {
    this._media = media;
  }

  get metadata() {
    return this._metadata;
  }

  set metadata(metadata) {
    this._metadata = metadata;
  }

  get modDate() {
    return this._modDate;
  }

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

  get mouseX() {
    return 0;
  }

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

  get mouseY() {
    return 0;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

Remove the assignment to `doc.modDate`; read it through the getter instead.
The modDate is updated by the authoring tool on save; edit there, not from script.
If you cannot edit the PDF's embedded script, wrap the statement in try/catch so the rest of the form logic still runs.

Example fix

// before
doc.modDate = new Date();
// after
// modDate is read-only — drop the assignment; read via doc.modDate

Defensive patterns

Strategy: validation

Validate before calling

// Known read-only doc properties (PDF.js scripting_api/doc.js)
const READONLY_DOC_PROPS = new Set([
  'author','bookmarkRoot','creator','dataObjects','docID',
  'documentFileName','dynamicXFAForm','external','filesize','hidden',
  'hostContainer','icons','info','innerAppWindowRect','innerDocWindowRect',
  'isModal','keywords','modDate'
]);
if (READONLY_DOC_PROPS.has('modDate')) {
  // skip the write; modDate is read-only in pdf.js
  console.warn('doc.modDate is read-only in pdf.js');
} else {
  doc.modDate = value;
}

Type guard

// Known read-only doc properties (PDF.js scripting_api/doc.js)
const READONLY_DOC_PROPS = new Set([
  'author','bookmarkRoot','creator','dataObjects','docID',
  'documentFileName','dynamicXFAForm','external','filesize','hidden',
  'hostContainer','icons','info','innerAppWindowRect','innerDocWindowRect',
  'isModal','keywords','modDate'
]);
const isReadOnlyDocProp = (name) => READONLY_DOC_PROPS.has(name);
// usage: if (!isReadOnlyDocProp('keywords')) doc.keywords = v;

Try / catch

try {
  doc.modDate = value;
} catch (e) {
  // pdf.js throws Error('doc.modDate is read-only')
  if (/is read-only/.test(e.message)) { /* swallow, expected */ }
  else { throw e; }
}

Prevention

Treat every property listed in the Acrobat JS reference as read-only unless pdf.js provides a setter that stores the value.
Before assigning, check `isReadOnlyDocProp('modDate')` or grep the setter in src/scripting_api/doc.js for a `throw`.
When porting scripts from Acrobat, run them against the pdf.js sandbox in the dev server first to surface writes early.
Prefer reading metadata through the Info dictionary at PDF authoring time rather than mutating it from a script.

When it happens

Trigger: A sandboxed PDF form/script executes an assignment to the property, e.g. `doc.modDate = value;` or `doc.modDate++`. Because the setter always throws (there is no condition), any write attempt triggers it.

Common situations: Scripts ported from desktop Acrobat where modDate was refreshed; forms that try to stamp runtime state into document metadata on save/open; and PDFs generated by tools that emit non-spec-compliant JavaScript.

Related errors

AI-assisted analysis of mozilla/pdf.js@5903d58d58 (2026-08-13). Data as JSON: /api/errors/9f43c8bbde4d343d. Report an issue: GitHub.