Error Fix: Cannot read properties of undefined (reading 'createElement') in a PDF.js Web Worker

TLDR

Invoking page.render() inside a Web Worker thread crashes with TypeError: Cannot read properties of undefined (reading 'createElement') because PDF.js defaults to DOMCanvasFactory, which calls document.createElement('canvas'). To resolve this inside workers, pass CanvasFactory: OffscreenCanvasFactory (capital 'C', class constructor) in getDocument() options.

getDocument() Option Passing Form	Evaluation Result	Web Worker Execution Outcome
canvasFactory: { create() {...} }	Ignored (Lowercase key ignored)	Crashes (document.createElement error)
CanvasFactory: new Factory()	Ignored / Throws (new applied to instance)	Crashes (Factory is not a constructor)
CanvasFactory: OffscreenCanvasFactory	Passes (new creates class instance)	Passes ($100\%$ Worker execution success)

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Defect diagnostics & source analysis

Inside PDF.js source code:

// PDF.js internal constructor lookup
const CanvasFactory = src.CanvasFactory || DefaultCanvasFactory;
// PDF.js instantiates factory internally this._canvasFactory = new CanvasFactory({ ownerDocument, enableHWA });

If src.CanvasFactory is omitted, PDF.js falls back to DefaultCanvasFactory (DOMCanvasFactory), calling document.createElement('canvas') and failing inside workers.

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Remediation code pattern

Implement OffscreenCanvasFactory and gate initialization on OffscreenCanvas availability:

export class OffscreenCanvasFactory {
  create(width, height) {
    const canvas = new OffscreenCanvas(width, height);
    return { canvas, context: canvas.getContext('2d') };
  }
reset(canvasAndCtx, width, height) {     canvasAndCtx.canvas.width = width;     canvasAndCtx.canvas.height = height;   }
destroy(canvasAndCtx) {     canvasAndCtx.canvas.width = 0;     canvasAndCtx.canvas.height = 0;     canvasAndCtx.canvas = null;     canvasAndCtx.context = null;   } }
export async function loadPdfDocumentInWorker(pdfBytes) {   const canvasFactoryOption = typeof OffscreenCanvas !== 'undefined'     ? { CanvasFactory: OffscreenCanvasFactory }     : {};
return pdfjsLib.getDocument({     data: pdfBytes,     ...canvasFactoryOption   }).promise; }

Rule of thumb: Pass CanvasFactory as an uninstantiated class constructor to PDF.js when initializing rendering inside Web Workers.