How to Create a File Object in JavaScript

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#1

How can I instantiate a File object in JavaScript? I’m aware that there’s a File object available, but I want to create one for testing purposes. However, when I try to use new File(), I receive an “Illegal constructor” error. Is it possible to create a javascript file object?

#2

I’ve worked with JavaScript for quite some time, and one effective way to create a JavaScript File Object is by utilizing the Blob constructor. It’s a straightforward approach. Here’s a quick example:


const blob = new Blob(['Hello, world!'], { type: 'text/plain' });

const file = new File([blob], 'hello.txt', { type: 'text/plain' });

console.log(file); // This is your JavaScript File Object

Using Blob in this way gives you more control over the content, allowing you to pass in data and specify file type details.

#3

That’s a great explanation! Building on Ishrath’s point, if your environment supports it, you can skip the Blob step and create a JavaScript File Object directly with the File constructor. It’s efficient and neat:


const file = new File(['Hello, world!'], 'hello.txt', { type: 'text/plain' });

console.log(file); // This is your JavaScript File Object

This method directly leverages the File constructor, making it easy to add both content and metadata.

#4

Those are solid techniques! Now, for a more interactive approach, especially when dealing with user-uploaded files, you can create a JavaScript File Object using a file input element in your HTML. Here’s a snippet:

const input = document.getElementById('fileInput');
input.addEventListener('change', (event) => {
    const file = event.target.files[0]; // This is your JavaScript File Object
    console.log(file);
});

This way, you can capture the actual file uploaded by the user, which is especially useful in real-world scenarios like form submissions.