JavaScript FileReader returns "undefined"

I was trying to Base64 encode a user-provided file with JS. This is my code:

let image = document.getElementById('image').files[0];
if (!image) {
    alert('No images have been provided!');
    return;
}
let blob = new Blob([image], { type: 'image/png' });
let reader = new FileReader;
reader.addEventListener('load', loadEvent => {
    let base64EncodedImage = reader.result
        .replace('data:', '');
});

Problem is that reader.readAsDataURL(blob) always returns undefined.