problem description

request a third-party interface, and use node, express and http-proxy-middleware to build a server to forward the request.
the front end sends the request using fetch. Some interfaces are accessible, while others are still prompted to cross-domain.

Q:

Why is this the case? if you request the server to forward the data directly in the browser, you can get the data normally. But sending a request with fetch prompts you to cross-domain.

some configurations of fetch:

const getFetch =(url)=> {
    try {
      let result = fetch(url, {
        // credentials: "omit",
        // "Content-Type": "application/json;charset=utf-8",
        // headers: {
        //   "Access-Control-Allow-Origin": "*",
          // Accept: "application/json, text/plain, */*"
          // "Content-Type": "application/json;charset=utf-8"
        // },
        // mode: "cors" 
      })
      return result.then(res=>res.json());
    }catch(err){
      console.error(err)
    }
}

const postFetch =(url,data)=> {

  let obj2params = (obj) => {

    let item, result = ""

    for ( item in obj ) {
      result += "&" + item + "=" + encodeURIComponent(obj[item]);
    }
    if (result) {
      result = result.slice(1);
    }
    return result;
  }
  
  try {
    let result = fetch(url, {
      method: "POST",
      credentials: "omit", 
      headers: {
        "Accept": "application/json, text/plain, */*",
        "Content-Type": "application/json"
      },
      // body: obj2params(data)
      body: JSON.stringify(data)
    });
    return result.then(res=>res.json());

  }catch(err) {
    console.error(err);
  }
}

browser normal request