Asynchronously read the contents of a user-uploaded file. A Promise wrapper around the FileReader API. IE 10+.

read-file.js

/**
 * Asynchronously read a file. This is simply a wrapper around the FileReader
 * API, which is not great.
 * @param {File|Blob} file to read
 * @return {Promise}
 */
export const readFile = file => {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();

    if (!(file instanceof Blob)) {
      const err = new Error('file must be an instance of File or Blob.');
      return reject(err);
    }

    reader.onerror = reject;
    reader.onabort = reject;
    reader.onloadend = contents => resolve(reader.result, file);

    return reader.readAsText(file);
  });
};