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Blob

ArrayBuffer and views are a part of ECMA standard, a part of JavaScript.

In the browser, there are additional higher-level objects, described in File API, in particular Blob.

Blob consists of an optional string type (a MIME-type usually), plus blobParts – a sequence of other Blob objects, strings and BufferSources.

The constructor syntax is:

new Blob(blobParts, options);
  • blobParts is an array of Blob/BufferSource/String values.
  • options optional object:
    • type – blob type, usually MIME-type, e.g. image/png,
    • endings – whether to transform end-of-line to make the blob correspond to current OS newlines (\r\n or \n). By default "transparent" (do nothing), but also can be "native" (transform).

For example:

// create Blob from a string
let blob = new Blob(["<html>…</html>"], {type: 'text/html'});
// please note: the first argument must be an array [...]
// create Blob from a typed array and strings
let hello = new Uint8Array([72, 101, 108, 108, 111]); // "hello" in binary form

let blob = new Blob([hello, ' ', 'world'], {type: 'text/plain'});

We can extract blob slices with:

blob.slice([byteStart], [byteEnd], [contentType]);
  • byteStart – the starting byte, by default 0.
  • byteEnd – the last byte (exclusive, by default till the end).
  • contentType – the type of the new blob, by default the same as the source.

The arguments are similar to array.slice, negative numbers are allowed too.

Blobs are immutable

We can’t change data directly in a blob, but we can slice parts of blobs, create new blobs from them, mix them into a new blob and so on.

This behavior is similar to JavaScript strings: we can’t change a character in a string, but we can make a new corrected string.

Blob as URL

A Blob can be easily used as an URL for <a>, <img> or other tags, to show its contents.

Thanks to type, we can allso download/upload blobs, and it naturally becomes Content-Type in network requests.

Let’s start with a simple example. By
clicking on a link you download a dynamically-generated blob with hello world contents as a file:

<!-- download attribute forces the browser to download instead of navigating -->
<a download="hello.txt" href='#' id="link">Download</a>

<script>
let blob = new Blob(["Hello, world!"], {type: 'text/plain'});

link.href = URL.createObjectURL(blob);
</script>

We can also create a link dynamically in JavaScript and simulate a click by link.click(), then download starts automatically.

Here’s the similar code that causes user to download the dynamicallly created Blob, without any HTML:

let link = document.createElement('a');
link.download = 'hello.txt';

let blob = new Blob(['Hello, world!'], {type: 'text/plain'});

link.href = URL.createObjectURL(blob);

link.click();

URL.revokeObjectURL(link.href);

URL.createObjectURL takes a blob and creates an unique URL for it, in the form blob:<origin>/<uuid>.

That’s what the value of link.href looks like:

blob:https://javascript.info/1e67e00e-860d-40a5-89ae-6ab0cbee6273

The browser for each url generated by URL.createObjectURL stores an the url → blob mapping internally. So such urls are short, but allow to access the blob.

A generated url (and hence the link with it) is only valid within the current document, while it’s open. And it allows to reference the blob in <img>, <a>, basically any other object that expects an url.

There’s a side-effect though. While there’s an mapping for a blob, the blob itself resides in the memory. The browser can’t free it.

The mapping is automatically cleared on document unload, so blobs are freed then. But if an app is long-living, then that doesn’t happen soon.

So if we create an URL, that blob will hang in memory, even if not needed any more.

URL.revokeObjectURL(url) removes the reference from the internal mapping, thus allowing the blob to be deleted (if there are no other references), and the memory to be freed.

In the last example, we intend the blob to be used only once, for instant downloading, so we call URL.revokeObjectURL(link.href) immediately.

In the previous example though, with the clickable HTML-link, we don’t call URL.revokeObjectURL(link.href), because that would make the blob url invalid. After the revocation, as the mapping is removed, the url doesn’t work any more.

Blob to base64

An alternative to URL.createObjectURL is to convert a blob into a base64-encoded string.

That encoding represents binary data as a string of ultra-safe “readable” characters with ASCII-codes from 0 to 64. And what’s more important – we can use this encoding in “data-urls”.

A data url has the form data:[<mediatype>][;base64],<data>. We can use such urls everywhere, on par with “regular” urls.

For instance, here’s a smiley:

<img src="data:image/png;base64,R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA7">

The browser will decode the string and show the image:

To transform a blob into base64, we’ll use the built-in FileReader object. It can read data from Blobs in multiple formats. In the next chapter we’ll cover it more in-depth.

Here’s the demo of downloading a blob, now via base-64:

let link = document.createElement('a');
link.download = 'hello.txt';

let blob = new Blob(['Hello, world!'], {type: 'text/plain'});

let reader = new FileReader();
reader.readAsDataURL(blob); // converts the blob to base64 and calls onload

reader.onload = function() {
  link.href = reader.result; // data url
  link.click();
};

Both ways of making an URL of a blob are usable. But usually URL.createObjectURL(blob) is simpler and faster.

URL.createObjectURL(blob)
  • We need to revoke them if care about memory.
  • Direct access to blob, no “encoding/decoding”
Blob to data url
  • No need to revoke anything.
  • Performance and memory losses on big blobs for encoding.

Image to blob

We can create a blob of an image, an image part, or even make a page screenshot. That’s handy to upload it somewhere.

Image operations are done via <canvas> element:

  1. Draw an image (or its part) on canvas using canvas.drawImage.
  2. Call canvas method .toBlob(callback, format, quality) that creates a blob and runs callback with it when done.

In the example below, an image is just copied, but we could cut from it, or transform it on canvas prior to making a blob:

// take any image
let img = document.querySelector('img');

// make <canvas> of the same size
let canvas = document.createElement('canvas');
canvas.width = img.clientWidth;
canvas.height = img.clientHeight;

let context = canvas.getContext('2d');

// copy image to it (this method allows to cut image)
context.drawImage(img, 0, 0);
// we can context.rotate(), and do many other things on canvas

// toBlob is async opereation, callback is called when done
canvas.toBlob(function(blob) {
  // blob ready, download it
  let link = document.createElement('a');
  link.download = 'example.png';

  link.href = URL.createObjectURL(blob);
  link.click();

  // delete the internal blob reference, to let the browser clear memory from it
  URL.revokeObjectURL(link.href);
}, 'image/png');

If we prefer async/await instead of callbacks:

let blob = await new Promise(resolve => canvasElem.toBlob(resolve, 'image/png'));

For screenshotting a page, we can use a library such as https://github.com/niklasvh/html2canvas. What it does is just walks the page and draws it on <canvas>. Then we can get a blob of it the same way as above.

From Blob to ArrayBuffer

The Blob constructor allows to create a blob from almost anything, including any BufferSource.

But if we need to perform low-level processing, we can get the lowest-level ArrayBuffer from it using FileReader:

// get arrayBuffer from blob
let fileReader = new FileReader();

fileReader.readAsArrayBuffer(blob);

fileReader.onload = function(event) {
  let arrayBuffer = fileReader.result;
};

Summary

While ArrayBuffer, Uint8Array and other BufferSource are “binary data”, a Blob represents “binary data with type”.

That makes Blobs convenient for upload/download operations, that are so common in the browser.

Methods that perform web-requests, such as XMLHttpRequest, fetch and so on, can work with Blob natively, as well as with other binary types.

We can easily convert betweeen Blob and low-level binary data types:

  • We can make a Blob from a typed array using new Blob(...) constructor.
  • We can get back ArrayBuffer from a Blob using FileReader, and then create a view over it for low-level binary processing.
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