What is the difference between file upload using FileReader and FormData?

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There are two ways I can upload files using Ajax (XHR2). First, I can read the file content as array buffer or binary string and then simply stream using XHR send method. For example, as shown here:

function uploadFile(img, file) {
  const reader = new FileReader();
  const xhr = new XMLHttpRequest();

  xhr.upload.addEventListener("progress", function(e) {
        if (e.lengthComputable) {
          const percentage = Math.round((e.loaded * 100) / e.total);
          // Do something with percentage
        }
      });

  xhr.upload.addEventListener("load", (e) => console.log('Do something more'));

  xhr.open("POST", "some-url");
  xhr.overrideMimeType('text/plain; charset=x-user-defined-binary');

  reader.onload = function(evt) {
    xhr.send(evt.target.result);
  };
  reader.readAsBinaryString(file);
}

Second, I can use FormData to upload my file as shown here:

var formData = new FormData();

// HTML file input, chosen by user
formData.append("userfile", fileInputElement.files[0]);

var request = new XMLHttpRequest();

request.open("POST", "some-url");

request.send(formData);

Are the two methods equivalent? Is there any advantage of using FileReader instead of FormData? Is one more performant than the other?

filereader ajax javascript
1 Answers

First, there is a third option you omitted which is to send the File directly through xhr.send(file) just like you did with the ArrayBuffer.

That being said, there doesn't exist any possible advantage to first reading the file in memory through FileReader.

When doing a file upload from a File on disk, the browser doesn't load the full file in memory but streams it through the request. This is how you can upload gigs of data even though it wouldn't fit in memory. This also is more friendly with the HDD since it allows for other processes to access it between each chunk instead of locking it.

When reading the File through a FileReader you are asking the browser to read the full file to memory, and then when you send it through XHR the data from memory is being used. You are thus limited by the memory available, bloating it for no good reasons, and even asking the CPU to work here while the data could have gone from the disk to the network card almost directly.

As to what's the difference between formdata.append(file); xhr.send(formdata); and xhr.send(file), basically only request headers. The former will wrap the request as a multipart/form-data enctype request, while the latter will send it as is.
So you'd handle both requests differently on the receiving end.

user_37027970
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