# Parse, serialize, manipulate, and sniff MIME types

This package will parse [MIME types](https://mimesniff.spec.whatwg.org/#understanding-mime-types) into a structured format, which can then be manipulated and serialized:

```js
const { MIMEType } = require("whatwg-mimetype");

const mimeType = new MIMEType(`Text/HTML;Charset="utf-8"`);

console.assert(mimeType.toString() === "text/html;charset=utf-8");

console.assert(mimeType.type === "text");
console.assert(mimeType.subtype === "html");
console.assert(mimeType.essence === "text/html");
console.assert(mimeType.parameters.get("charset") === "utf-8");

mimeType.parameters.set("charset", "windows-1252");
console.assert(mimeType.parameters.get("charset") === "windows-1252");
console.assert(mimeType.toString() === "text/html;charset=windows-1252");

console.assert(mimeType.isHTML() === true);
console.assert(mimeType.isXML() === false);
```

It can also [determine the MIME type of a resource](https://mimesniff.spec.whatwg.org/#determining-the-computed-mime-type-of-a-resource) by sniffing its contents:

```js
const { computedMIMEType } = require("whatwg-mimetype");

const pngBytes = new Uint8Array([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]);

computedMIMEType(pngBytes).essence;                                    // "image/png"
computedMIMEType(pngBytes, { contentTypeHeader: "image/gif" }).essence // "image/png"
computedMIMEType(pngBytes, { contentTypeHeader: "text/html" }).essence // "text/html"
```

Parsing, serialization, and sniffing are all fairly complex processes; see the specification for more details:

* [parsing](https://mimesniff.spec.whatwg.org/#parsing-a-mime-type)
* [serialization](https://mimesniff.spec.whatwg.org/#serializing-a-mime-type)
* [sniffing](https://mimesniff.spec.whatwg.org/#determining-the-computed-mime-type-of-a-resource)

This package's algorithms conform to those of the WHATWG [MIME Sniffing Standard](https://mimesniff.spec.whatwg.org/), as of [commit `e7594d5`](https://github.com/whatwg/mimesniff/commit/e7594d531819053508447f688a3bde465531aca5).

## APIs

### `MIMEType`

The `MIMEType` class's constructor takes a string which it will attempt to parse into a MIME type; if parsing fails, an `Error` will be thrown.

#### The `parse()` static factory method

As an alternative to the constructor, you can use `MIMEType.parse(string)`. The only difference is that `parse()` will return `null` on failed parsing, whereas the constructor will throw. It thus makes the most sense to use the constructor in cases where unparseable MIME types would be exceptional, and use `parse()` when dealing with input from some unconstrained source.

#### Properties

* `type`: the MIME type's [type](https://mimesniff.spec.whatwg.org/#mime-type-type), e.g. `"text"`
* `subtype`: the MIME type's [subtype](https://mimesniff.spec.whatwg.org/#mime-type-subtype), e.g. `"html"`
* `essence`: the MIME type's [essence](https://mimesniff.spec.whatwg.org/#mime-type-essence), e.g. `"text/html"`
* `parameters`: an instance of `MIMETypeParameters`, containing this MIME type's [parameters](https://mimesniff.spec.whatwg.org/#mime-type-parameters)

`type` and `subtype` can be changed. They will be validated to be non-empty and only contain [HTTP token code points](https://mimesniff.spec.whatwg.org/#http-token-code-point).

`essence` is only a getter, and cannot be changed.

`parameters` is also a getter, but the contents of the `MIMETypeParameters` object are mutable, as described below.

#### Methods

* `toString()` serializes the MIME type to a string
* `isHTML()`: returns true if this instance represents [a HTML MIME type](https://mimesniff.spec.whatwg.org/#html-mime-type)
* `isXML()`: returns true if this instance represents [an XML MIME type](https://mimesniff.spec.whatwg.org/#xml-mime-type)
* `isJavaScript({ prohibitParameters })`: returns true if this instance represents [a JavaScript MIME type](https://html.spec.whatwg.org/multipage/scripting.html#javascript-mime-type). `prohibitParameters` can be set to true to disallow any parameters, i.e. to test if the MIME type's serialization is a [JavaScript MIME type essence match](https://mimesniff.spec.whatwg.org/#javascript-mime-type-essence-match).

_Note: the `isHTML()`, `isXML()`, and `isJavaScript()` methods are speculative, and may be removed or changed in future major versions. See [whatwg/mimesniff#48](https://github.com/whatwg/mimesniff/issues/48) for brainstorming in this area. Currently we implement these mainly because they are useful in jsdom._

### `MIMETypeParameters`

The `MIMETypeParameters` class, instances of which are returned by `mimeType.parameters`, has equivalent surface API to a [JavaScript `Map`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map).

However, `MIMETypeParameters` methods will always interpret their arguments as appropriate for MIME types, so e.g. parameter names will be lowercased, and attempting to set invalid characters will throw.

Some examples:

```js
const { MIMEType } = require("whatwg-mimetype");

const mimeType = new MIMEType(`x/x;a=b;c=D;E="F"`);

// Logs:
// a b
// c D
// e F
for (const [name, value] of mimeType.parameters) {
  console.log(name, value);
}

console.assert(mimeType.parameters.has("a"));
console.assert(mimeType.parameters.has("A"));
console.assert(mimeType.parameters.get("A") === "b");

mimeType.parameters.set("Q", "X");
console.assert(mimeType.parameters.get("q") === "X");
console.assert(mimeType.toString() === "x/x;a=b;c=d;e=F;q=X");

// Throws:
mimeType.parameters.set("@", "x");
```

### `computedMIMEType()`

The `computedMIMEType(resource, options)` function determines a resource's MIME type using the full [MIME type sniffing algorithm](https://mimesniff.spec.whatwg.org/#determining-the-computed-mime-type-of-a-resource). This includes:

* complexities around how the supplied `Content-Type` header or other external information interacts with (but usually does not override) the sniffing process;
* the ability to set a "no-sniff" flag, like the one used when `X-Content-Type-Options: nosniff` is present, which mostly (but not entirely) prevents sniffing; and
* the "Apache bug" check which occurs when the `Content-Type` header is one of several specific values.

That is, this doesn't just implement the [rules for identifying an unknown MIME type](https://mimesniff.spec.whatwg.org/#rules-for-identifying-an-unknown-mime-type). It gives you everything you need for a full browser-compatible MIME sniffing procedure.

#### Arguments

* **`resource`** (`Uint8Array`): The resource bytes.
* **`options.contentTypeHeader`** (`string`): The Content-Type header value, for HTTP resources.
* **`options.providedType`** (`string`): The MIME type from the filesystem or another protocol (like FTP), for non-HTTP resources.
* **`options.noSniff`** (`boolean`, default `false`): Whether the `X-Content-Type-Options: nosniff` header was present.
* **`options.isSupported`** (`function`, default `() => true`): A predicate called with a `MIMEType` instance to check if an image/audio/video MIME type is supported by the user agent. If it returns `false`, sniffing is skipped for that type.

The `contentTypeHeader` and `providedType` options will be stringified, so you could also supply a `MIMEType` (or a [Node.js `util` module `MIMEType`](https://nodejs.org/api/util.html#class-utilmimetype)).

#### Return value

A `MIMEType` instance representing the computed MIME type.