9º aprile 2020
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File and FileReader

A File object inherits from Blob and is extended with filesystem-related capabilities.

There are two ways to obtain it.

First, there’s a constructor, similar to Blob:

new File(fileParts, fileName, [options])
  • fileParts – is an array of Blob/BufferSource/String values.
  • fileName – file name string.
  • options – optional object:
    • lastModified – the timestamp (integer date) of last modification.

Second, more often we get a file from <input type="file"> or drag’n’drop or other browser interfaces. In that case, the file gets this information from OS.

As File inherits from Blob, File objects have the same properties, plus:

  • name – the file name,
  • lastModified – the timestamp of last modification.

That’s how we can get a File object from <input type="file">:

<input type="file" onchange="showFile(this)">

function showFile(input) {
  let file = input.files[0];

  alert(`File name: ${file.name}`); // e.g my.png
  alert(`Last modified: ${file.lastModified}`); // e.g 1552830408824
Da notare:

The input may select multiple files, so input.files is an array-like object with them. Here we have only one file, so we just take input.files[0].


FileReader is an object with the sole purpose of reading data from Blob (and hence File too) objects.

It delivers the data using events, as reading from disk may take time.

The constructor:

let reader = new FileReader(); // no arguments

The main methods:

  • readAsArrayBuffer(blob) – read the data in binary format ArrayBuffer.
  • readAsText(blob, [encoding]) – read the data as a text string with the given encoding (utf-8 by default).
  • readAsDataURL(blob) – read the binary data and encode it as base64 data url.
  • abort() – cancel the operation.

The choice of read* method depends on which format we prefer, how we’re going to use the data.

  • readAsArrayBuffer – for binary files, to do low-level binary operations. For high-level operations, like slicing, File inherits from Blob, so we can call them directly, without reading.
  • readAsText – for text files, when we’d like to get a string.
  • readAsDataURL – when we’d like to use this data in src for img or another tag. There’s an alternative to reading a file for that, as discussed in chapter Blob: URL.createObjectURL(file).

As the reading proceeds, there are events:

  • loadstart – loading started.
  • progress – occurs during reading.
  • load – no errors, reading complete.
  • abortabort() called.
  • error – error has occurred.
  • loadend – reading finished with either success or failure.

When the reading is finished, we can access the result as:

  • reader.result is the result (if successful)
  • reader.error is the error (if failed).

The most widely used events are for sure load and error.

Here’s an example of reading a file:

<input type="file" onchange="readFile(this)">

function readFile(input) {
  let file = input.files[0];

  let reader = new FileReader();


  reader.onload = function() {

  reader.onerror = function() {

FileReader for blobs

As mentioned in the chapter Blob, FileReader can read not just files, but any blobs.

We can use it to convert a blob to another format:

  • readAsArrayBuffer(blob) – to ArrayBuffer,
  • readAsText(blob, [encoding]) – to string (an alternative to TextDecoder),
  • readAsDataURL(blob) – to base64 data url.
FileReaderSync is available inside Web Workers

For Web Workers, there also exists a synchronous variant of FileReader, called FileReaderSync.

Its reading methods read* do not generate events, but rather return a result, as regular functions do.

That’s only inside a Web Worker though, because delays in synchronous calls, that are possible while reading from files, in Web Workers are less important. They do not affect the page.


File objects inherit from Blob.

In addition to Blob methods and properties, File objects also have name and lastModified properties, plus the internal ability to read from filesystem. We usually get File objects from user input, like <input> or Drag’n’Drop events (ondragend).

FileReader objects can read from a file or a blob, in one of three formats:

  • String (readAsText).
  • ArrayBuffer (readAsArrayBuffer).
  • Data url, base-64 encoded (readAsDataURL).

In many cases though, we don’t have to read the file contents. Just as we did with blobs, we can create a short url with URL.createObjectURL(file) and assign it to <a> or <img>. This way the file can be downloaded or shown up as an image, as a part of canvas etc.

And if we’re going to send a File over a network, that’s also easy: network API like XMLHttpRequest or fetch natively accepts File objects.

Mappa del tutorial


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