212 lines
8.4 KiB
Markdown
212 lines
8.4 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/).
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`Blob` consists of an optional string `type` (a MIME-type usually), plus `blobParts` this a sequence of other `Blob` objects, strings and `BufferSources`.
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Thanks to `type`, we can download/upload blobs, and it naturally becomes `Content-Type` in network requests.
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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 `\r\n` to `\n` end vise versa, to make the blob correspond to current OS newlines. By default `"transparent"` (do nothing), but also can be `"native"` (transform).
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Then we can access blob as a whole, or extract 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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We can't change data directly in a blob, but we can copy parts of blobs, create new blobs from them, mix them 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.
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## Blob as URL
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A Blob can be easily used in an URL for `<a>`, `<img>` or other tags.
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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 binary = new Uint8Array([72, 101, 108, 108, 111]); // "hello" in binary form
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// create blob of multiple parts
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let blob = new Blob([binary, ' ', '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 in Javascript and simulate a click by `link.click()`, then download starts authomatically.
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Here's the similar "on the fly" blob creation and download code, now without 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 an unique URL for it, in the form `blob:<origin>/<uuid>`.**
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That's what the generated url 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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The browser keeps an internal url -> blob mapping. For each url generated by `URL.createObjectURL`, it stores a reference to the corresponding blob.
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A generated url is only valid while the current document is open. And it allows to reference the blob in `<img>`, `<a>`, any other object that expects an URL.
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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.
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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. So if we create an 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 though, with the clickable HTML-link, we dont'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 and vise-versa, we'll use built-in `FileReader/FileWriter` objects. In the [next chapter](info:file) we'll cover those more in-detail. For now, it's important only that they can read a blob as data url, that we can assign it to `link.href`.
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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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*/!*
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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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reader.readAsDataURL(blob); // converts the blob to base64 and calls onload
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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 blobs for encoding.
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```
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## Image or a page to blob
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We can create a blob of an image, or even a page screenshot.
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That's usually done via `<canvas>` element:
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1. We draw an existing image on canvas using [canvas.drawImage](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawImage).
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2. Canvas [.toBlob(callback, format, quality)](https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob) creates a blob and calls `callback` with it.
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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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let img = document.querySelector('img'); // just any image
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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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context.drawImage(img, 0, 0); // allows to cut image
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// we can context.rotate() as well, and do many other things
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canvas.toBlob(function(blob) {
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// download the blob
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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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URL.revokeObjectURL(link.href);
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}, '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> to draw the screenshot on `<canvas>`, and then download it the same way.
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## From Blob to ArrayBuffer
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The `Blob` constructor allows to create a blob from almost anything.
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But we can also revert back to lowest-level `ArrayBuffer` 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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fileReader.onload = function(event) {
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let arrayBuffer = fileReader.result;
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};
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*!*
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fileReader.readAsArrayBuffer(blob);
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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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That said, it's still possible to 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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