Javascript 获取中的自定义标头未命中服务器

当我创建header对象并传递它时，它不工作，甚至没有击中服务器

const post = async (url, request) => {
let testHeader = new Headers();
testHeader.append('Accept', 'application/json');
testHeader.append('Content-Type', 'application/json');
const response = await fetch(url,
    {
        method: 'POST',
        credentials: 'same-origin',
        headers: testHeader,
        body: JSON.stringify(request)
    }
);
return responseHandler(request, response);
};

但是当我像这样传递标题时，它解决了任何问题

const post = async (url, request) => {
const response = await fetch(url,
    {
        method: 'POST',
        credentials: 'same-origin',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        body: JSON.stringify(request)
    }
  );
  return responseHandler(request, response);
  };