Uncaught (in promise) SyntaxError: Unexpected token T in JSON at position 0

Question

When I try send an array of data to the frontend, it works fine, but when I try to send a sample text to the frontend from the server, getting the error Unexpected token T in JSON at position 0. I am getting this error on console.log(data).

fetch("/something", {
    method: "POST",
    headers: {'Content-Type': 'application/json'}, 
    body: JSON.stringify(data)
  }).then(function (response) {
    // The API call was successful!
    console.log(response);
    return response.json();
  }).then(function (data) {
    // This is the JSON from our response
    console.log(data);
 
  });

router.post('/something', () => {
  console.log(req.body);
    res.send('This is sample');
});

Answer 1

This is because res.send() automatically sets the Content-Type response header as well based on the argument passed to the send() method. In your case, send() is not detecting a json object as an argument. So, the content type is not json in the response header.

You could use res.json() method to send the string as json.

router.post('/something', () => {
  console.log(req.body);
    res.json('This is sample');
});

Or, pass a javascript object to res.send() and it would automatically convert it into json.

router.post('/something', () => {
  console.log(req.body);
    res.send({data: 'This is sample'});
});

Note: res.json() uses res.send() under the hood. Read this article to understand the difference between the two methods.