Express API 只返回对象的第一个键值对

Question

I'm fetching nested object data. I can get that object data on console but when I try to get that data like return res.json(imageObject) I only get first key value pair of that object. This is the error to the console. UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client . Console data is like this { bigImage: 'https://url.com' }

 router.get("/", async(req, res) => { //...fetch data if (mediaData.type === "type1") { let allData = await mediaData.media; allData.map(async(Data) => { if (Data.imageType === "big") { let bigImage = await Data.url; let imageObject = { bigImage }; console.log(imageObject); return res.json(imageObject); } }); } });

Answer 1

You are using res inside a .map , which will trigger it once for every item in the array you're iterating.

You can't do that. You can only use res once for each req , because you're replying to the browser. If you use it more than once, you get this error, which means it's already been used and you have already replied.

Solution : use res once, not inside a map .

Also, .map is useless here, because you're not interested in the result it produces. You should use forEach or even better, a for loop (which is breakable on condition).

Answer 2

You should use for..of .

It will stop the execution of your function at the return statement

router.get("/", async(req, res) => {
  //...fetch data
  if (mediaData.type === "type1") {
    let allData = await mediaData.media;

    let pendingResult = allData.filter(data => data.imageType === "big").map(async item => {
       let bigImage = await item.url;
       return { bigImage }
    });
    
    let result = await Promise.all(pendingResult);

    res.json(result);
  }
});