Browser JS: File <-> Base64 String

base64file.ts

function fileToBase64(file: File): Promise<string | ArrayBuffer | null> {
  return new Promise((resolve, reject) => {
    const r = new FileReader()
    r.onload = () => resolve(r.result)
    r.onerror = (err) => reject(err)
    r.readAsDataURL(file)
  })
}

async function base64ToFile(str: string, fileName: string = "newfile.txt", type: string = "text/plain"): Promise<File> {
  const res = await fetch(str)
  const blob = await res.blob()
  return new File([blob], fileName, { type })
}