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

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

mozilla/pdf.js · error · Error

doc.hostContainer is read-only

Error message

doc.hostContainer 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. `hostContainer` is the host-container object for message passing; pdf.js returns `undefined` (not implemented) and forbids writes.

Source

Thrown at src/scripting_api/doc.js:386

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

  get hidden() {
    return false;
  }

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

  get hostContainer() {
    return undefined;
  }

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

  get icons() {
    return undefined;
  }

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

  get info() {
    return this._info;
  }

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

View on GitHub (pinned to 5903d58d58)

Solutions

Remove the assignment to `doc.hostContainer`; read it through the getter instead.
Use the host page's postMessage API outside the sandbox instead; pdf.js does not implement hostContainer.
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.hostContainer = {};
// after
// hostContainer is read-only — drop the assignment; read via doc.hostContainer

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('hostContainer')) {
  // skip the write; hostContainer is read-only in pdf.js
  console.warn('doc.hostContainer is read-only in pdf.js');
} else {
  doc.hostContainer = 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.hostContainer = value;
} catch (e) {
  // pdf.js throws Error('doc.hostContainer 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('hostContainer')` 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.hostContainer = value;` or `doc.hostContainer++`. Because the setter always throws (there is no condition), any write attempt triggers it.

Common situations: Scripts ported from desktop Acrobat where hostContainer was set; 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/5c41cc4b5a1ef756. Report an issue: GitHub.