# HTTP cache semantics

`CachePolicy` object that computes properties of a HTTP response, such as whether it's fresh or stale, and how long it can be cached for. Based on RFC 7234.

## Usage

```js
const cache = new CachePolicy(request, response, options);

// Age counts from the time response has been created
const secondsFresh = cache.maxAge();
const secondsOld = cache.age();

// Current state
const outOfDate = cache.stale();
```

Cacheability of response depends on how it was requested, so both request and response are required. Both are objects with `headers` property that is an object with lowercased header names as keys, e.g.

```js
const request = {
    method: 'GET',
    headers: {
        'accept': '*/*',
    },
};

const response = {
    status: 200,
    headers: {
        'cache-control': 'public, max-age=7234',
    },
};

const options = {
    shared: true,
};
```

If `options.shared` is true (default), then response is evaluated from perspective of a shared cache (i.e. `private` is not cacheable and `s-maxage` is respected). If `options.shared` is false, then response is evaluated from perspective of a single-user cache (i.e. `private` is cacheable and `s-maxage` is ignored).

### `stale()`

Returns `true` if the response is stale (i.e. not fresh).

It generally means the response can't be used any more without revalidation with the server. However, there are exceptions, e.g. client can explicitly allow stale responses. A fresh response still may not be used if other conditions—such as `Vary`—are not satisfied.

### `cacheKey()`

Returns a string that is a combination of method, URL, and headers selected with `Vary`.

Note that `Vary: *` never matches any request, so matching of cache keys alone is not sufficient to satisfy a request.

## Implemented

* `Expires` with check for bad clocks
* `Cache-Control` response header
* `Pragma` response header
* `Age` response header
* Default cacheability of statuses and methods
* Basic support for `Vary`

## Unimplemented

* No support for revalidation and stale responses