react native fetch and convert file to base64

rn-to-base64.ts

interface FileReader {
  readAsDataURL(blob: Blob): string;
  onloadend(): void;
  result: string;
}

declare var FileReader: {
  prototype: FileReader;
  new (): FileReader;
};

function toDataURL(url: string, callback: (uri: string) => void) {
  const xhr = new XMLHttpRequest();
  xhr.withCredentials = true;
  xhr.onload = function() {
    const reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    };
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send(null);
}