Filereader example

index.js

const fileSelector = document.getElementById('file-selector')

fileSelector.addEventListener('change', (event) => {
    const file = event.target.files[0]    
    const filereader = new FileReader()

    filereader.onloadend = function (evt) {
        if (evt.target.readyState === FileReader.DONE) {
            const uint = new Uint8Array(evt.target.result)
            let bytes = []
            uint.forEach((byte) => {
                bytes.push(byte.toString(16))
            })
            const hex = bytes.join('').toUpperCase()
            console.log(hex)
        }
    }
    const blob = file.slice(0, 4);
    filereader.readAsArrayBuffer(blob);
})