How to use FormData and send multipart-data in the Browser (Web API )?

About

FormData is a web api object that represent the data of a form that should be send if the form is submited.

Therefore if you have a checkbox that is not checked, formdata will not collect it.

You can create/build it and :

send it in a Http request (XMLHttpRequest.send() method)
use it in a form validation

As javascript object, you will get a set of key/value pairs representing form fields and their values.

When send, the data is encoded in multipart/form-data encoding ¹⁾

It's part of the XHR specification ²⁾

Example

How to send a form data

via Ajax XHR: You can also just send it via a Xhr request

var request = new XMLHttpRequest();
request.open("POST", "http://example.com/api");
request.send(formData);

via fetch Api

let formData = new FormData(form);
fetch("/combo/api/v1.0/publication/subscription", {
     body: formData,
     cache: 'no-cache',
     method: "post",
     mode: 'no-cors',
     redirect: 'follow',
     credentials: 'same-origin'
  })
  .then(response => console.log(response))

Build from the form

The form (Return false to prevent the default action of the submit event - ie to not get a 404)

<form id="form1" onsubmit="handleSubmit(this); return false;">
<input name="user" type="text" value="Foo" />
<input name="age" type="number" value="10" />
<input name="button" type="submit" value="Let mee see the input data !" />
</form>

The event handler that will handle the submit event and loop over the entries iterator

The FormData constructor takes a form element as parameter. See FormData constructor Specification

handleSubmit = (form) => {
  // build the formData object
  let formData =  new FormData(form);
  // retrieve the entries in a entries variable of iterator type
  let entries = formData.entries();
  // loop over the iterator
  let result = entries.next();
  while (!result.done) {
    console.log("Input: Name: "+result.value[0]+", value: "+result.value[1]);
    result = entries.next();
  }
}

Result:

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Management

Create

Manually

let formData = new FormData();
formData.append('username', 'foo');
formData.append('age', '10');
formData.append('tag', 'cool');
formData.append('tag', 'bar');

// You can't get all entries but you can get all values for one key
console.log(formData.getAll("tag").length);

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Form

From a form, only successful form controls are included, i.e. those:

with a name,
not disabled
checked for radio buttons and checkboxes.
or selected one or more options within a select.

let formData =  new FormData(document.getElementById('form1'));

From a key-value object

const formData = new FormData();
Object.keys(data).forEach(key => formData.append(key, data[key]));

Read

As iterator

The entries of a formData return an iterator.

Example:

let formData = new FormData();
formData.append('username', 'foo');
formData.append('age', '10');
// The entries function returns an iterator
let it = formData.entries();

The loop (iteration)

let result = it.next();
while (!result.done) {
 console.log(result.value); 
 result = it.next();
}

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As an Object

const formJson = Object.fromEntries(formData.entries());

As an array of key-value pair

[...formData.entries()]

Browser Devtool

You can see it nicely formatted in the BrowserDevtool

Event

The formdata event is fired when:

a form is constructing the entry list of the form data after the submit event. You could then add data in the list.
a new FormData() constructor is created.

Example:

<form id="form1" onSubmit="handleSubmit(this); return false;">
<p><label>FirstName</label> <input name="firstname" type="text" value="FirstNameDefault"/></p>
<p><input name="button" type="submit" value="Let mee see the formDataEvent !" /></p>
</form>

The FormData cannot cancel the action of a submit. Therefore, we take over the submit event, prevent it and build ourself the formdata.

let handleSubmit = (form) => {
    console.log('submit event was fired');
    // build the form data to fire the FormDataEvent on the form
    new FormData(form);
}

The event handler that will handle the formdata event

document.addEventListener("formdata", (formDataEvent) => {
    
    console.log('formdata event was fired');

    // Get the form data from the event object via values
    let formData = formDataEvent.formData;
    console.log("entries:");
    for (var entry of formData ) {
        console.log(entry[0]+" : "+entry[1]);
    }
    console.log("");

    // Get the form data from the event object via get
    console.log("Input is present ? : "+formData.has("firstname"));
    console.log("Value via the name attribute is: "+formData.get("firstname"));
    
    // You can also
    // delete(name) 
    // getAll(name) (for multiple)
    // set(name, value) and set(name, blobValue, filename)
    
    // submit further the data via XHR
    // let request = new XMLHttpRequest();
    // request.open("POST", "/formHandler");
    // request.send(data);
});

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Construction Process

The formData is populated by following this procedure: constructing-the-form-data-set

Documentation / Reference

¹⁾

As see in the extracting body procedure where they run the multipart/form-data encoding algorithm, with object’s entry list

²⁾

interface-formdata XHR specification