Read the body of a Fetch Promise

Question

I have the following express endpoint for uploading to Google Cloud storage. It works great and the response from the google api gives me a unique file name that I want to pass back to my front end:

app.post('/upload', (req, res) => {
  var form = new formidable.IncomingForm(),
  files = [],
  fields = [];

  form
    .on('field', function(field, value) {
      fields.push([field, value]);
    })
    .on('file', function(field, file) {
      files.push([field, file]);
    })
    .on('end', function() {
      console.log('-> upload done');
    });
  form.parse(req, function(err, fields, files){
    var filePath = files.file.path;
    bucket.upload(filePath, function(err, file, apiResponse){
      if (!err){
        res.writeHead(200, {'content-type': 'text/plain'});
        res.end("Unique File Name:" + file.name);
      }else{
        res.writeHead(500);
        res.end();
      }
    });
  });

 return;
});

I reach this endpoint by calling a short function which passes the file to it:

function upload(file) {
  var data = new FormData();
  data.append('file', file);
  return fetch(`upload`,{
    method: 'POST',
    body: data
  });
}

const Client = { upload };
export default Client;

This function is called from my front end like this:

Client.upload(this.file).then((data) => {
  console.log(data);
});

This final console.log(data) logs the response to the console. However, I don't see anywhere the response that I wrote in ("Unique File Name:" + file.name)

How I can retrieve this info from the response body on the client-side?

The data looks like this when I console.log it:

This is the response I get when I POST a file to my endpoint using Postman:

Answer 1

Notice you're dealing with a Response object. You need to basically read the response stream with Response.json() or Response.text() (or via other methods) in order to see your data. Otherwise your response body will always appear as a locked readable stream. For example:

fetch('https://api.ipify.org?format=json')
.then(response=>response.json())
.then(data=>{ console.log(data); })

If this gives you unexpected results, you may want to inspect your response with Postman.

Answer 2

I had a typo in my code as pointed out by GabeRogan in this comment:

Ok awesome. To be quite honest I have absolutely no clue why you're getting undefined, except that it might be some sort of misspelling error?

Here's my updated code for the front end which returns the response body text:

Client.upload(this.file).then(response => response.text())
  .then((body) => {
    console.log(body);
  });

body is a string that reads Unique File Name: [FILE-NAME]

Here's a good explanation of the Fetch API and reading the response you get from the promise object: Css tricks: Using Fetch.

Answer 3

You can also use async/await:

When returning json content:

Client.upload(this.file).then(async r => console.log(await r.json()))

or just returning in textual form:

Client.upload(this.file).then(async r => console.log(await r.text()))

Answer 4

If you are getting "undefined" for data and you are doing something with the response, make sure to return the response.

I was doing something like this

fetch(URL)
    .then(response => {
        response.text();
        console.log(response.statusText);
    })
    .then(data => console.log(data)); // got "undefined"

Return response object: return response.text();

fetch(URL)
    .then(response => {
        console.log(response.statusText);
        return response.text();
    })
    .then(data => console.log(data)); // got data