在 string 转成ArrayBuffer报错

public static transStringToAB(str: string): ArrayBuffer { 
  if (str === "") { 
    return new ArrayBuffer(0); // 返回一个空的ArrayBuffer 
  } else { 
    const encoder = new util.TextEncoder(); 
    const encoded = encoder.encode(str); 
    return encoded.buffer; 
  } 
}