How to upload a file using Fetch

There’s a task that should be simple, but sometimes it leads to hours of research on the Web: uploading files to the server.

In this tutorial I explain you how to do so using fetch.

Given a form with a file 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.

We initialize a new FormData object and we assign it to the formData variable, and we append there the uploaded file. If we have more than one file input element, we’d have more than one append() call.

The data variable inside the second then() will contain the JSON parsed return data. I’m assuming your server will give you JSON as a response.

How to handle images uploaded server-side is a different topic, one I’ll write about tomorrow.