How do I post form data with fetch api?

To quote MDN on FormData (emphasis mine):

The FormData interface provides a way to easily construct a set of key/value pairs representing form fields and their values, which can then be easily sent using the XMLHttpRequest.send() method. It uses the same format a form would use if the encoding type were set to "multipart/form-data".

So when using FormData you are locking yourself into multipart/form-data. There is no way to send a FormData object as the body and not sending data in the multipart/form-data format.

If you want to send the data as application/x-www-form-urlencoded you will either have to specify the body as an URL-encoded string, or pass a URLSearchParams object. The latter unfortunately cannot be directly initialized from a form element. If you don’t want to iterate through your form elements yourself (which you could do using HTMLFormElement.elements), you could also create a URLSearchParams object from a FormData object:

const data = new URLSearchParams();
for (const pair of new FormData(formElement)) {
    data.append(pair[0], pair[1]);
}

fetch(url, {
    method: 'post',
    body: data,
})
.then(…);

Note that you do not need to specify a Content-Type header yourself.

As noted by monk-time in the comments, you can also create URLSearchParams and pass the FormData object directly, instead of appending the values in a loop:

const data = new URLSearchParams(new FormData(formElement));

This still has some experimental support in browsers though, so make sure to test this properly before you use it.

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Client

Do not set the content-type header.

// Build formData object.
let formData = new FormData();
formData.append('name', 'John');
formData.append('password', 'John123');

fetch("api/SampleData",
    {
        body: formData,
        method: "post"
    });

Server

Use the FromForm attribute to specify that binding source is form data.

[Route("api/[controller]")]
public class SampleDataController : Controller
{
    [HttpPost]
    public IActionResult Create([FromForm]UserDto dto)
    {
        return Ok();
    }
}

public class UserDto
{
    public string Name { get; set; }
    public string Password { get; set; }
}

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Use FormData and fetch to grab and send data

fetch(form.action, {method:'post', body: new FormData(form)});

function send(e,form) { fetch(form.action, {method:'post', body: new FormData(form)}); console.log('We send post asynchronously (AJAX)'); e.preventDefault(); }

Look on chrome console>network before/after 'submit'

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You can set body to an instance of URLSearchParams with query string passed as argument

fetch("/path/to/server", {
  method:"POST"
, body:new URLSearchParams("email=test@example.com&password=pw")
})

document.forms[0].onsubmit = async(e) => { e.preventDefault(); const params = new URLSearchParams([...new FormData(e.target).entries()]); // fetch("/path/to/server", {method:"POST", body:params}) const response = await new Response(params).text(); console.log(response); }

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With fetch api it turned out that you do NOT have to include headers "Content-type": "multipart/form-data".

So the following works:

let formData = new FormData()
formData.append("nameField", fileToSend)

fetch(yourUrlToPost, {
   method: "POST",
   body: formData
})

Note that with axios I had to use the content-type.

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👨‍💻These can help you:

let formData = new FormData();
            formData.append("name", "John");
            formData.append("password", "John123");
            fetch("https://yourwebhook", {
              method: "POST",
              mode: "no-cors",
              cache: "no-cache",
              credentials: "same-origin",
              headers: {
                "Content-Type": "form-data"
              },
              body: formData
            });
            //router.push("/registro-completado");
          } else {
            // doc.data() will be undefined in this case
            console.log("No such document!");
          }
        })
        .catch(function(error) {
          console.log("Error getting document:", error);
        });

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To add on the good answers above you can also avoid setting explicitly the action in HTML and use an event handler in javascript, using "this" as the form to create the "FormData" object

Html form :

<form id="mainForm" class="" novalidate>
<!--Whatever here...-->
</form>

In your JS :

$("#mainForm").submit(function( event ) {
  event.preventDefault();
  const formData = new URLSearchParams(new FormData(this));
  fetch("http://localhost:8080/your/server",
    {   method: 'POST',
        mode : 'same-origin',
        credentials: 'same-origin' ,
        body : formData
    })
    .then(function(response) {
      return response.text()
    }).then(function(text) {
        //text is the server's response
    });
});

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@KamilKiełczewski answer is great if you are okay with the form data format being in form multipart style, however if you need the form submitted in query parameter styles:

You can also pass FormData directly to the URLSearchParams constructor if you want to generate query parameters in the way a would do if it were using simple GET submission.

        form = document.querySelector('form')
        const formData = new FormData(form);
        formData["foo"] = "bar";
        const payload = new URLSearchParams(formData)
        fetch(form.action, payload)

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"body:FormData" works but there're type complains, also "FormData" sets multipart headers. To make the things simplier, "body:URLSearchParams" with inline construction and headers set manually may be used :

function getAccessToken(code) {

    return fetch(tokenURL, 
        {
            method: 'POST',
            headers: {
               'Content-Type': 'application/x-www-form-urlencoded',                 
               'Accept': '*/*' 
            },            
            body: new URLSearchParams({
                'client_id':clientId,    
                'client_secret':clientSecret,
                'code':code,    
                'grant_type': grantType,
                'redirect_uri':'',
                'scope':scope
            })
        }
        )
    .then(
        r => return r.json()
    ).then(
        r => r.access_token
    )
  }

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There are instructions on the MDN that the browser will automatically handle Content-Type.

[A request will also automatically set a Content-Type header if none is set in the dictionary.]（https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#body）

So we don't need to specify 'content-type' when we send a fetch request.

const formData = new FormData();
const fileField = document.querySelector('input[type="file"]');

formData.append('username', 'abc123');
formData.append('avatar', fileField.files[0]);

fetch('https://example.com/profile/avatar', {
  method: 'PUT',
  body: formData
})
.then(response => response.json())
.then(result => {
  console.log('Success:', result);
})
.catch(error => {
  console.error('Error:', error);
});

If set content-type in headers. Browser will not try to split formdata in request payload.

I'm using fathcer to handle FormData, the same behavior as XHR.

import { formData } from '@fatcherjs/middleware-form-data';
import { json } from '@fatcherjs/middleware-json';
import { fatcher } from 'fatcher';

fatcher({
    url: '/bar/foo',
    middlewares: [json(), formData()],
    method: 'PUT',
    payload: {
        bar: 'foo',
        file: new File()
    },
    headers: {
        'Content-Type': 'multipart/form-data',
    },
})
    .then(res => {
        console.log(res);
    })
    .catch(err => {
        console.error(error);
    });

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To post form data with fetch api, try this code it works for me ^_^

function card(fileUri) {
let body = new FormData();
let formData = new FormData();
formData.append('file', fileUri);

fetch("http://X.X.X.X:PORT/upload",
  {
      body: formData,
      method: "post"
  });
 }

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