241 lines
9.3 KiB
Markdown
241 lines
9.3 KiB
Markdown
# Blob
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`ArrayBuffer` and views are a part of ECMA standard, a part of JavaScript.
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In the browser, there are additional higher-level objects, described in [File API](https://www.w3.org/TR/FileAPI/), in particular `Blob`.
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`Blob` consists of an optional string `type` (a MIME-type usually), plus `blobParts` -- a sequence of other `Blob` objects, strings and `BufferSource`.
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The constructor syntax is:
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```js
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new Blob(blobParts, options);
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```
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- **`blobParts`** is an array of `Blob`/`BufferSource`/`String` values.
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- **`options`** optional object:
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- **`type`** -- `Blob` type, usually MIME-type, e.g. `image/png`,
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- **`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).
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For example:
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```js
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// create Blob from a string
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let blob = new Blob(["<html>…</html>"], {type: 'text/html'});
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// please note: the first argument must be an array [...]
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```
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```js
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// create Blob from a typed array and strings
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let hello = new Uint8Array([72, 101, 108, 108, 111]); // "Hello" in binary form
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let blob = new Blob([hello, ' ', 'world'], {type: 'text/plain'});
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```
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We can extract `Blob` slices with:
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```js
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blob.slice([byteStart], [byteEnd], [contentType]);
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```
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- **`byteStart`** -- the starting byte, by default 0.
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- **`byteEnd`** -- the last byte (exclusive, by default till the end).
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- **`contentType`** -- the `type` of the new blob, by default the same as the source.
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The arguments are similar to `array.slice`, negative numbers are allowed too.
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```smart header="`Blob` objects are immutable"
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We can't change data directly in a `Blob`, but we can slice parts of a `Blob`, create new `Blob` objects from them, mix them into a new `Blob` and so on.
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This behavior is similar to JavaScript strings: we can't change a character in a string, but we can make a new corrected string.
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```
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## Blob as URL
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A Blob can be easily used as an URL for `<a>`, `<img>` or other tags, to show its contents.
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Thanks to `type`, we can also download/upload `Blob` objects, and the `type` naturally becomes `Content-Type` in network requests.
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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:
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```html run
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<!-- download attribute forces the browser to download instead of navigating -->
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<a download="hello.txt" href='#' id="link">Download</a>
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<script>
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let blob = new Blob(["Hello, world!"], {type: 'text/plain'});
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link.href = URL.createObjectURL(blob);
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</script>
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```
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We can also create a link dynamically in JavaScript and simulate a click by `link.click()`, then download starts automatically.
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Here's the similar code that causes user to download the dynamicallly created `Blob`, without any HTML:
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```js run
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let link = document.createElement('a');
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link.download = 'hello.txt';
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let blob = new Blob(['Hello, world!'], {type: 'text/plain'});
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link.href = URL.createObjectURL(blob);
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link.click();
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URL.revokeObjectURL(link.href);
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```
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`URL.createObjectURL` takes a `Blob` and creates a unique URL for it, in the form `blob:<origin>/<uuid>`.
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That's what the value of `link.href` looks like:
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```
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blob:https://javascript.info/1e67e00e-860d-40a5-89ae-6ab0cbee6273
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```
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For each URL generated by `URL.createObjectURL` the browser stores a URL -> `Blob` mapping internally. So such URLs are short, but allow to access the `Blob`.
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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.
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There's a side-effect though. While there's a mapping for a `Blob`, the `Blob` itself resides in the memory. The browser can't free it.
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The mapping is automatically cleared on document unload, so `Blob` objects are freed then. But if an app is long-living, then that doesn't happen soon.
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**So if we create a URL, that `Blob` will hang in memory, even if not needed any more.**
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`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.
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In the last example, we intend the `Blob` to be used only once, for instant downloading, so we call `URL.revokeObjectURL(link.href)` immediately.
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In the previous example 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.
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## Blob to base64
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An alternative to `URL.createObjectURL` is to convert a `Blob` into a base64-encoded string.
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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".
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A [data url](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs) has the form `data:[<mediatype>][;base64],<data>`. We can use such urls everywhere, on par with "regular" urls.
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For instance, here's a smiley:
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```html
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<img src="data:image/png;base64,R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA7">
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```
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The browser will decode the string and show the image: <img src="data:image/png;base64,R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA7">
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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](info:file) we'll cover it more in-depth.
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Here's the demo of downloading a blob, now via base-64:
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```js run
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let link = document.createElement('a');
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link.download = 'hello.txt';
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let blob = new Blob(['Hello, world!'], {type: 'text/plain'});
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*!*
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let reader = new FileReader();
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reader.readAsDataURL(blob); // converts the blob to base64 and calls onload
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*/!*
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reader.onload = function() {
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link.href = reader.result; // data url
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link.click();
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};
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```
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Both ways of making an URL of a `Blob` are usable. But usually `URL.createObjectURL(blob)` is simpler and faster.
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```compare title-plus="URL.createObjectURL(blob)" title-minus="Blob to data url"
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+ We need to revoke them if care about memory.
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+ Direct access to blob, no "encoding/decoding"
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- No need to revoke anything.
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- Performance and memory losses on big `Blob` objects for encoding.
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```
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## Image to blob
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We can create a `Blob` of an image, an image part, or even make a page screenshot. That's handy to upload it somewhere.
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Image operations are done via `<canvas>` element:
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1. Draw an image (or its part) on canvas using [canvas.drawImage](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawImage).
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2. Call canvas method [.toBlob(callback, format, quality)](https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob) that creates a `Blob` and runs `callback` with it when done.
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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:
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```js run
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// take any image
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let img = document.querySelector('img');
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// make <canvas> of the same size
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let canvas = document.createElement('canvas');
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canvas.width = img.clientWidth;
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canvas.height = img.clientHeight;
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let context = canvas.getContext('2d');
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// copy image to it (this method allows to cut image)
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context.drawImage(img, 0, 0);
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// we can context.rotate(), and do many other things on canvas
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// toBlob is async opereation, callback is called when done
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canvas.toBlob(function(blob) {
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// blob ready, download it
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let link = document.createElement('a');
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link.download = 'example.png';
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link.href = URL.createObjectURL(blob);
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link.click();
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// delete the internal blob reference, to let the browser clear memory from it
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URL.revokeObjectURL(link.href);
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}, 'image/png');
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```
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If we prefer `async/await` instead of callbacks:
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```js
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let blob = await new Promise(resolve => canvasElem.toBlob(resolve, 'image/png'));
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```
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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.
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## From Blob to ArrayBuffer
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The `Blob` constructor allows to create a blob from almost anything, including any `BufferSource`.
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But if we need to perform low-level processing, we can get the lowest-level `ArrayBuffer` from it using `FileReader`:
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```js
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// get arrayBuffer from blob
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let fileReader = new FileReader();
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*!*
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fileReader.readAsArrayBuffer(blob);
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*/!*
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fileReader.onload = function(event) {
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let arrayBuffer = fileReader.result;
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};
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```
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## Summary
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While `ArrayBuffer`, `Uint8Array` and other `BufferSource` are "binary data", a [Blob](https://www.w3.org/TR/FileAPI/#dfn-Blob) represents "binary data with type".
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That makes Blobs convenient for upload/download operations, that are so common in the browser.
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Methods that perform web-requests, such as [XMLHttpRequest](info:xmlhttprequest), [fetch](info:fetch) and so on, can work with `Blob` natively, as well as with other binary types.
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We can easily convert betweeen `Blob` and low-level binary data types:
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- We can make a Blob from a typed array using `new Blob(...)` constructor.
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- 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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