利用Promise.race为fetch提供缺失的timeout的功能，Promise.race的作用是挑选出进行后续then操作的promise，其实每个promise都会被执行，不要在其中做具有side effects的操作

fetch_timeout.js

function timeoutPromise(timeout) {
  return new Promise((resolve, reject) => {
    setTimeout(resolve, timeout);
  });
}

request(url, options, timeout) {
  const defaultOptions = {
    credentials: 'include',
  };
  const newOptions = { ...defaultOptions, ...options };
  if (newOptions.method === 'POST' || newOptions.method === 'PUT') {
    if (!(newOptions.body instanceof FormData)) {
      newOptions.headers = {
        Accept: 'application/json',
        'Content-Type': 'application/json; charset=utf-8',
        ...newOptions.headers,
      };
      newOptions.body = JSON.stringify(newOptions.body);
    } else {
      // newOptions.body is FormData
      newOptions.headers = {
        Accept: 'application/json',
        ...newOptions.headers,
      };
    }
  }
  
  return Promise.race([
    fetch(url, newOptions),
    timeoutPromise(timeout).then(() => {
      throw new Error('timeout');
    })
  ])
    .then(response => {
      if (newOptions.method === 'DELETE' || response.status === 204) {
        return response.text();
      }
      return response.json();
    })
    .catch(err => {
      return {
        success: false,
        message: err.message,
      };
    });
}