# Context

  A Koa Context encapsulates node's `request` and `response` objects
  into a single object which provides many helpful methods for writing
  web applications and APIs.
  These operations are used so frequently in HTTP server development
  that they are added at this level instead of a higher level framework,
  which would force middleware to re-implement this common functionality.

  A `Context` is created _per_ request, and is referenced in middleware
  as the receiver, or the `ctx` identifier, as shown in the following
  snippet:

```js
app.use(async ctx => {
  ctx; // is the Context
  ctx.request; // is a Koa Request
  ctx.response; // is a Koa Response
});
```

  Many of the context's accessors and methods simply delegate to their `ctx.request` or `ctx.response`
  equivalents for convenience, and are otherwise identical. For example `ctx.type` and `ctx.length`
  delegate to the `response` object, and `ctx.path` and `ctx.method` delegate to the `request`.

## API

  `Context` specific methods and accessors.

### ctx.req

  Node's `request` object.

### ctx.res

  Node's `response` object.

  Bypassing Koa's response handling is __not supported__. Avoid using the following node properties:

- `res.statusCode`
- `res.writeHead()`
- `res.write()`
- `res.end()`

### ctx.request

  A Koa `Request` object.

### ctx.response

  A Koa `Response` object.

### ctx.state

  The recommended namespace for passing information through middleware and to your frontend views.

```js
ctx.state.user = await User.find(id);
```

### ctx.app

  Application instance reference.

### ctx.app.emit

  Koa applications extend an internal [EventEmitter](https://nodejs.org/dist/latest-v11.x/docs/api/events.html). `ctx.app.emit` emits an event with a type, defined by the first argument. For each event you can hook up "listeners", which is a function that is called when the event is emitted. Consult the [error handling docs](https://koajs.com/#error-handling) for more information.

### ctx.throw([status], [msg], [properties])

  Helper method to throw an error with a `.status` property
  defaulting to `500` that will allow Koa to respond appropriately.
  The following combinations are allowed:

```js
ctx.throw(400);
ctx.throw(400, 'name required');
ctx.throw(400, 'name required', { user: user });
```

  For example `ctx.throw(400, 'name required')` is equivalent to:

```js
const err = new Error('name required');
err.status = 400;
err.expose = true;
throw err;
```

  Note that these are user-level errors and are flagged with
  `err.expose` meaning the messages are appropriate for
  client responses, which is typically not the case for
  error messages since you do not want to leak failure
  details.

  You may optionally pass a `properties` object which is merged into the error as-is, useful for decorating machine-friendly errors which are reported to the requester upstream.

```js
ctx.throw(401, 'access_denied', { user: user });
```

Koa uses [http-errors](https://github.com/jshttp/http-errors) to create errors. `status` should only be passed as the first parameter.

### ctx.assert(value, [status], [msg], [properties])

  Helper method to throw an error similar to `.throw()`
  when `!value`. Similar to node's [assert()](http://nodejs.org/api/assert.html)
  method.

```js
ctx.assert(ctx.state.user, 401, 'User not found. Please login!');
```

Koa uses [http-assert](https://github.com/jshttp/http-assert) for assertions.

### ctx.respond

  To bypass Koa's built-in response handling, you may explicitly set `ctx.respond = false;`. Use this if you want to write to the raw `res` object instead of letting Koa handle the response for you.

  Note that using this is __not__ supported by Koa. This may break intended functionality of Koa middleware and Koa itself. Using this property is considered a hack and is only a convenience to those wishing to use traditional `fn(req, res)` functions and middleware within Koa.

## Request aliases

  The following accessors and alias [Request](request.md) equivalents:

  - `ctx.header`
  - `ctx.headers`
  - `ctx.method`
  - `ctx.method=`
  - `ctx.url`
  - `ctx.url=`
  - `ctx.originalUrl`
  - `ctx.origin`
  - `ctx.href`
  - `ctx.path`
  - `ctx.path=`
  - `ctx.query`
  - `ctx.query=`
  - `ctx.querystring`
  - `ctx.querystring=`
  - `ctx.host`
  - `ctx.hostname`
  - `ctx.fresh`
  - `ctx.stale`
  - `ctx.socket`
  - `ctx.protocol`
  - `ctx.secure`
  - `ctx.ip`
  - `ctx.ips`
  - `ctx.subdomains`
  - `ctx.is()`
  - `ctx.get()`

## Response aliases

  The following accessors and alias [Response](response.md) equivalents:

  - `ctx.body`
  - `ctx.body=`
  - `ctx.status`
  - `ctx.status=`
  - `ctx.message`
  - `ctx.message=`
  - `ctx.length=`
  - `ctx.length`
  - `ctx.type=`
  - `ctx.type`
  - `ctx.headerSent`
  - `ctx.redirect()`
  - `ctx.attachment()`
  - `ctx.set()`
  - `ctx.append()`
  - `ctx.remove()`
  - `ctx.lastModified=`
  - `ctx.etag=`