How to Use FileReaderSync in JavaScript Web Workers

The FileReaderSync API provides a straightforward way to read File or Blob objects synchronously, returning the file data directly to the execution context. Because synchronous file input/output blocks code execution until the operation completes, this interface is exclusively available inside JavaScript Web Workers. This article explains how FileReaderSync works, why it is restricted to background threads, the methods it provides, and how to implement it with practical code examples.

What is FileReaderSync?

In standard browser environments, the asynchronous FileReader interface uses event listeners (onload, onerror) or promises to read files without freezing the user interface. Conversely, FileReaderSync reads files sequentially and blocks execution within the worker until the entire file content is loaded into memory.

By eliminating callback chains and event listeners, FileReaderSync simplifies algorithmic flows—especially when performing sequential binary processing, image manipulation, or parsing large data files.

Why Is It Restricted to Web Workers?

JavaScript in the browser runs on a single main execution thread responsible for handling user interactions, animations, and rendering. If a synchronous file read were executed on the main thread, the entire webpage would freeze until the file was completely read from disk.

Web Workers operate on a separate background thread. Blocking a Web Worker with a synchronous operation has zero impact on the responsiveness of the main UI thread, making synchronous reading safe within this isolated context.

Available Methods

The FileReaderSync interface provides four primary methods, each returning the read data directly rather than emitting an event:

• readAsArrayBuffer(blob): Reads the specified Blob or File and returns the data as an ArrayBuffer. This is ideal for binary data processing and WebAssembly integration.
• readAsText(blob, [encoding]): Reads the content and returns it as a plain text string. An optional encoding parameter (such as 'UTF-8') can be provided.
• readAsDataURL(blob): Reads the file and returns a base64-encoded Data URL string.
• readAsBinaryString(blob): Reads the file and returns the raw binary data as a string (note: this method is deprecated in favor of readAsArrayBuffer).

Implementation Example

To use FileReaderSync, pass a File or Blob reference from the main thread to a dedicated Web Worker via postMessage.

1. Main Thread (main.js)

// Initialize the worker
const worker = new Worker('worker.js');

// Handle the file input change event
document.getElementById('fileInput').addEventListener('change', (event) => {
  const file = event.target.files[0];
  if (file) {
    // Send the File object to the worker
    worker.postMessage(file);
  }
});

// Receive the processed data from the worker
worker.onmessage = (event) => {
  console.log('File content received from worker:', event.data);
};

2. Worker Thread (worker.js)

self.onmessage = function (event) {
  const file = event.data;

  // Instantiate FileReaderSync
  const reader = new FileReaderSync();

  try {
    // Synchronously read the file as text
    const textContent = reader.readAsText(file);

    // Perform any heavy synchronous parsing or processing here
    const processedData = textContent.toUpperCase();

    // Send the result back to the main thread
    self.postMessage(processedData);
  } catch (error) {
    // Synchronous reads throw standard exceptions on failure
    self.postMessage({ error: error.message });
  }
};

Error Handling

Unlike the asynchronous FileReader, which requires listening to the error event, FileReaderSync throws standard JavaScript exceptions when a read operation fails (e.g., due to file access restrictions or missing files). Wrap calls to FileReaderSync methods inside standard try...catch blocks to catch and handle DOMException errors properly.