Blob.size

The Blob interface's size property returns the size of the Blob or File in bytes.

var sizeInBytes = blob.size

The number of bytes of data contained within the Blob (or Blob-based object, such as a File).

This example uses an <input> element of type file to ask the user for a group of files, then iterates over those files outputting their names and lengths in bytes.

// fileInput is a HTMLInputElement: <input type="file" multiple id="myfileinput">
var fileInput = document.getElementById("myfileinput");

// files is a FileList object (simliar to NodeList)
var files = fileInput.files;

for (var i = 0; i < files.length; i++) {
  console.log(files[i].name + " has a size of " + files[i].size + " Bytes");
}

• Blob
• Using files from web applications