How to use js to determine whether a file is utf-8 encoded

const isUtf8 = async (file: File) => {
  return await new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsText(file);

    reader.onloadend = (e: any): void => {
      const content = e.target.result;
      const encodingRight = content.indexOf("") === -1;

      if (encodingRight) {
        resolve(encodingRight);
      } else {
        reject(new Error("Encoding format error, please upload UTF-8 format file"));
      }
    };
    
    reader.onerror = () => {
      reject(new Error("File content reading failed, please check if the file is damaged"));
    };
  });
};

const getSamples = (file: File) => {
  const filesize = file.size;
  const parts: Blob[] = [];
  if (filesize < 50 * 1024 * 1024) {
    parts.push(file);
  } else {
    let total = 100;
    const sampleSize = 1024 * 1024;
    const chunkSize = Math.floor(filesize / total);
    let start = 0;
    let end = sampleSize;
    while (total > 1) {
      parts.push(file.slice(start, end));
      start += chunkSize;
      end += chunkSize;
      total--;
    }
  }
  return parts;
};

const isUtf8 = (filePart: Blob) => {
  return new Promise((resolve, reject) => {
    const fileReader = new FileReader();

    fileReader.readAsText(filePart);

    fileReader.onload = (e) => {
      const str = e.target?.result as string;
      // Take roughly half const sampleStr = str?.slice(4, 4 + str?.length / 2);
      if (sampleStr.indexOf("�") === -1) {
        resolve(void 0);
      } else {
        reject(new Error(Encoding format error, please upload UTF-8 format file"));
      }
    };

    fileReader.onerror = () => {
      reject(new Error(File content reading failed, please check if the file is damaged"));
    };
  });
};

export default async function (file: File) {
  const samples = getSamples(file);
  let res = true;

  for (const filePart of samples) {
    try {
      await isUtf8(filePart);
    } catch (error) {
      res = false;
      break;
    }
  }
  return res;
}