Reading files in JavaScript with FileReader

Reading files in JavaScript with FileReader

In this article, we will explore how to use FileReader to read files in JavaScript.

JavaScript is a programming language widely used for developing interactive web applications. One of the most common operations in any web application is file management. FileReader is an embedded JavaScript object that allows you to read the contents of files from the user's local disk. In this article, we will explore how to use FileReader to read files in JavaScript.

Introduction to FileReader

FileReader is an object that is part of the HTML5 File API specification and is widely supported in modern browsers. It is primarily used to read files from a specified source, such as from an HTML file input or a drag-and-drop API. Once a file is uploaded, its contents can be accessed in various formats, such as text or binary data.

Using FileReader to read files

To use FileReader, follow these steps:

1. Create a FileReader object

First of all, create an instance of FileReader:

const fileReader = new FileReader();

2. Manage events

FileReader uses events to notify you when file loading has completed or if an error has occurred. The main events are onload and onerror. Here's how you can manage them:

fileReader.onload = function(event) {
   // The file has been read successfully, you can access its contents here.
   const fileContent =;

fileReader.onerror = function(event) {
   // An error occurred while reading the file.
   console.error("Error reading file:",;

3. Read the file

To read the contents of the file, use the readAsText() method if the file is text or readAsArrayBuffer() if it is binary:

const fileInput = document.getElementById('fileInput'); // Replace with the ID of your file input element

fileInput.addEventListener('change', function() {
   const selectedFile = fileInput.files[0];
   if (selectedFile) {
     // Read the file as text
     // If you want to read it as binary data, use instead:
     // fileReader.readAsArrayBuffer(selectedFile);

4. Use the contents of the file

Once the file has been successfully read, you can access its contents in the onload event handler. For example, if the file is text, you can simply access the content using, as shown in the previous example.


FileReader is a powerful API that makes reading files from a web application much easier. You can use it to allow users to load and view the contents of files within your application, thus opening up a wide range of possibilities for interacting with file data.