How to use the FormData object

Published Jun 05 2022

⚠️⚠️ JUST A FEW HOURS LEFT to JOIN THE 2023 BOOTCAMP ⚠️⚠️

The FormData object is used to store form input fields values.

It’s especially useful when you need to send files to the server.

It’s probably the only time you’ll actually need it.

Here is one example of using FormData to send the content of a file using fetch.

We have an input field:

<input type="file" id="fileUpload" />

We attach a change event handler on it:

document.querySelector('#fileUpload').addEventListener('change', event => {
  handleImageUpload(event)
})

and we manage the bulk of our logic in the handleImageUpload() function:

const handleImageUpload = event => {
  const files = event.target.files
  const formData = new FormData()
  formData.append('myFile', files[0])

  fetch('/saveImage', {
    method: 'POST',
    body: formData
  })
  .then(response => response.json())
  .then(data => {
    console.log(data)
  })
  .catch(error => {
    console.error(error)
  })
}

In this example we POST to the /saveImage endpoint.

You could send more data too by appending it to the formData object.

In the server-side, to access the file data you must parse the request as a multipart form.

See for example how to upload files in a Next.js form

The FormData object you create has many useful methods:

append() to add a value to the object, with the specified key. If the key already exists, the value is added to that key, without eliminating the first one
delete() deletes a key value pair
entries() gives an Iterator object you can loop to list the key value pairs hosted
get() get the value associated with a key. If more than one value was appended, it returns the first one
getAll() get all the values associated with a key
has() returns true if there’s a key
keys() gives an Iterator object you can loop to list the keys hosted
set() to add a value to the object, with the specified key. If the key already exists, the value is replaced
values() gives an Iterator object you can loop to list the values hosted