How do I post form data with fetch api?

My code:

fetch("api/xxx", {
    body: new FormData(document.getElementById("form")),
    headers: {
        "Content-Type": "application/x-www-form-urlencoded",
        // "Content-Type": "multipart/form-data",
    },
    method: "post",
}

I tried to post my form using fetch api, and the body it sends is like:

-----------------------------114782935826962
Content-Disposition: form-data; name="email"

test@example.com
-----------------------------114782935826962
Content-Disposition: form-data; name="password"

pw
-----------------------------114782935826962--

(I don't know why the number in boundary is changed every time it sends...)

I would like it to send the data with "Content-Type": "application/x-www-form-urlencoded", what should I do? Or if I just have to deal with it, how do I decode the data in my controller?

To whom answer my question, I know I can do it with:

fetch("api/xxx", {
    body: "[email protected]&password=pw",
    headers: {
        "Content-Type": "application/x-www-form-urlencoded",
    },
    method: "post",
}

What I want is something like $("#form").serialize() in jQuery (w/o using jQuery) or the way to decode mulitpart/form-data in controller. Thanks for your answers though.

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.

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; }
}

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 protected]&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);
}

<form>
  <input name="email" value="[email protected]">
  <input name="password" value="pw">
  <input type="submit">
</form>

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();
}

<form method="POST" action="myapi/send" onsubmit="send(event,this)">
    <input hidden name="csrfToken" value="a1e24s1">
    <input name="email" value="[email protected]">
    <input name="phone" value="123-456-789">
    <input type="submit">    
</form>

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

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.

‍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);
        });

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
	});
});

@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

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

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"
  });
 }