Convert data file to blob

How to get a blob?

HTML:

<input type="file" onchange="previewFile()">

JavaScript:

function previewFile() {
  var file    = document.querySelector('input[type=file]').files[0];
  var reader  = new FileReader();

  // Get blob?
  console.log(file);
}

As pointed in the comments, file is a blob:

file instanceof Blob; // true

And you can get its content with the file reader API https://developer.mozilla.org/en/docs/Web/API/FileReader

Read more: https://developer.mozilla.org/en-US/docs/Using_files_from_web_applications

var input = document.querySelector('input[type=file]');
var textarea = document.querySelector('textarea');

function readFile(event) {
  textarea.textContent = event.target.result;
  console.log(event.target.result);
}

function changeFile() {
  var file = input.files[0];
  var reader = new FileReader();
  reader.addEventListener('load', readFile);
  reader.readAsText(file);
}

input.addEventListener('change', changeFile);

<input type="file">
<textarea rows="10" cols="50"></textarea>

A file object is an instance of Blob but a blob object is not an instance of File

new File([], 'foo.txt').constructor.name === 'File' //true
new File([], 'foo.txt') instanceof File // true
new File([], 'foo.txt') instanceof Blob // true

new Blob([]).constructor.name === 'Blob' //true
new Blob([]) instanceof Blob //true
new Blob([]) instanceof File // false

new File([], 'foo.txt').constructor.name === new Blob([]).constructor.name //false

If you must convert a file object to a blob object, you can create a new Blob object using the array buffer of the file. See the example below.

const file = new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'});
//or const file = document.querySelector('input[type=file]').files[0];
const reader = new FileReader();
reader.onload = function(e) {
    const blob = new Blob([new Uint8Array(e.target.result)], {type: file.type });
    console.log(blob);
};
reader.readAsArrayBuffer(file);

As pointed out by @bgh you can also use the arrayBuffer method of the File object. See the example below.

const file = new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'});
//or const file = document.querySelector('input[type=file]').files[0];

file.arrayBuffer().then((arrayBuffer) => {
    const blob = new Blob([new Uint8Array(arrayBuffer)], {type: file.type });
    console.log(blob);
});

If your environment supports async/await you can use a one-liner like below

const fileToBlob = async (file) => new Blob([new Uint8Array(await file.arrayBuffer())], {type: file.type });
console.log(await fileToBlob(new File(['hello', ' ', 'world'], 'hello_world.txt', {type: 'text/plain'})));