The FormData object is used to hold form values in a key-value pairs data structure.

You create an empty FormData object in this way:

const fd = new FormData()

Once you have the item you can call one of its 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

The FormData object is part of the XMLHttpRequest 2 spec. It’s available in all the modern browsers but keep in mind that IE versions prior to 10 do not support it.

Here is one example of using FormData to send the content of a file using an XHR connection:

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

const sendFile = file => {
  const uri = '/saveImage'
  const xhr = new XMLHttpRequest()
  const fd = new FormData()

  xhr.open('POST', uri, true)
  xhr.onreadystatechange = () => {
    if (xhr.readyState == 4 && xhr.status == 200) {
      const imageName = xhr.responseText
      //do what you want with the image name returned
      //e.g update the interface
    }
  }
  fd.append('myFile', file)
  xhr.send(fd)
}

const handleImageUpload = event => {
  const files = event.target.files
  sendFile(files[0])
}

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

This snippet instead can be used to send multiple files:

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

const sendFiles = files => {
  const uri = '/saveImage'
  const xhr = new XMLHttpRequest()
  const fd = new FormData()

  xhr.open('POST', uri, true)
  xhr.onreadystatechange = () => {
    if (xhr.readyState == 4 && xhr.status == 200) {
      const imageName = xhr.responseText
      //do what you want with the image name returned
      //e.g update the interface
    }
  }

  for (let i = 0; i < files.length; i++) {
    fd.append(files[i].name, files[i])
  }

  xhr.send(fd)
}

const handleImageUpload = event => {
  sendFiles(event.target.files)
}

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

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