# 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](https://www.w3.org/TR/FileAPI/), in particular `Blob`. `Blob` consists of an optional string `type` (a MIME-type usually), plus `blobParts` -- a sequence of other `Blob` objects, strings and `BufferSource`. ![](blob.svg) The constructor syntax is: ```js 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: ```js // create Blob from a string let blob = new Blob(["…"], {type: 'text/html'}); // please note: the first argument must be an array [...] ``` ```js // 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: ```js 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. ```smart header="`Blob` objects are immutable" 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. 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 a URL for ``, `` or other tags, to show its contents. Thanks to `type`, we can also download/upload `Blob` objects, and the `type` 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: ```html run Download ``` 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 dynamically created `Blob`, without any HTML: ```js run 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 a unique URL for it, in the form `blob:/`. That's what the value of `link.href` looks like: ``` blob:https://javascript.info/1e67e00e-860d-40a5-89ae-6ab0cbee6273 ``` 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`. 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 ``, ``, basically any other object that expects a URL. 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. 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. **So if we create a 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 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](mdn:/http/Data_URIs) has the form `data:[][;base64],`. We can use such urls everywhere, on par with "regular" urls. For instance, here's a smiley: ```html ``` 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](info:file) we'll cover it more in-depth. Here's the demo of downloading a blob, now via base-64: ```js run 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 a URL of a `Blob` are usable. But usually `URL.createObjectURL(blob)` is simpler and faster. ```compare title-plus="URL.createObjectURL(blob)" title-minus="Blob to data url" + We need to revoke them if care about memory. + Direct access to blob, no "encoding/decoding" - No need to revoke anything. - Performance and memory losses on big `Blob` objects 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 `` element: 1. Draw an image (or its part) on canvas using [canvas.drawImage](mdn:/api/CanvasRenderingContext2D/drawImage). 2. Call canvas method [.toBlob(callback, format, quality)](mdn:/api/HTMLCanvasElement/toBlob) 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: ```js run // take any image let img = document.querySelector('img'); // make 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 operation, 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: ```js let blob = await new Promise(resolve => canvasElem.toBlob(resolve, 'image/png')); ``` For screenshotting a page, we can use a library such as . What it does is just walks the page and draws it on ``. 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 `blob.arrayBuffer()`: ```js // get arrayBuffer from blob const bufferPromise = await blob.arrayBuffer(); // or blob.arrayBuffer().then(buffer => /* process the ArrayBuffer */); ``` `FileReader.readAsArrayBuffer()` method can also get ArrayBuffer, but it usually fails if the blob memory exceeds 2G. It is recommended to use blob.arrayBuffer(), it's simpler. ## From Blob to stream When we read and write to a blob of more than `2G`, the use of `arrayBuffer` becomes more memory intensive for us. At this point, we can directly convert the blob to a `stream`，The `Blob` interface's `stream()` method returns a `ReadableStream` which upon reading returns the data contained within the `Blob`. ```js // get readableStream from blob const readableStream = blob.stream(); const reader = readableStream.getReader(); while (true) { // value => blob fragments let { done, value } = await reader.read(); if (done) { console.log('write done.'); break; } // to do sth stream.write(value); } ``` ## Summary While `ArrayBuffer`, `Uint8Array` and other `BufferSource` are "binary data", a [Blob](https://www.w3.org/TR/FileAPI/#dfn-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](info:xmlhttprequest), [fetch](info:fetch) and so on, can work with `Blob` natively, as well as with other binary types. We can easily convert between `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 `blob.arrayBuffer()`, and then create a view over it for low-level binary processing. Conversion streams are very useful when we need to handle large blob. You can easily create a `ReadableStream` from a blob. The `Blob` interface's `stream()` method returns a `ReadableStream` which upon reading returns the data contained within the blob.