koa-lite/docs/api/context.md

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# 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,
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which would force middleware to re-implement this common functionality.
A `Context` is created _per_ request, and is referenced in middleware
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as the receiver, or the `this` identifier, as shown in the following
snippet:
```js
app.use(function *(){
this; // is the Context
this.request; // is a koa Request
this.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
this.state.user = yield User.find(id);
```
### ctx.app
Application instance reference.
### ctx.cookies.get(name, [options])
Get cookie `name` with `options`:
- `signed` the cookie requested should be signed
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koa uses the [cookies](https://github.com/jed/cookies) module where options are simply passed.
### ctx.cookies.set(name, value, [options])
Set cookie `name` to `value` with `options`:
- `signed` sign the cookie value
- `expires` a `Date` for cookie expiration
- `path` cookie path, `/'` by default
- `domain` cookie domain
- `secure` secure cookie
- `httpOnly` server-accessible cookie, __true__ by default
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koa uses the [cookies](https://github.com/jed/cookies) module where options are simply passed.
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### ctx.throw([msg], [status], [properties])
Helper method to throw an error with a `.status` property
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defaulting to `500` that will allow Koa to respond appropriately.
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The following combinations are allowed:
```js
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this.throw(403);
this.throw('name required', 400);
this.throw(400, 'name required');
this.throw('something exploded');
```
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For example `this.throw('name required', 400)` is equivalent to:
```js
var err = new Error('name required');
err.status = 400;
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
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details.
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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.
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```js
this.throw(401, 'access_denied', { user: user });
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this.throw('access_denied', { user: user });
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```
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koa uses [http-errors](https://github.com/jshttp/http-errors) to create errors.
### ctx.assert(value, [msg], [status], [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
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this.assert(this.state.user, 401, 'User not found. Please login!');
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```
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 `this.respond = false;`. Use this if you want to write to the raw `res` object instead of letting Koa handle the response for you.
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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`
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- `ctx.headers`
- `ctx.method`
- `ctx.method=`
- `ctx.url`
- `ctx.url=`
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- `ctx.originalUrl`
- `ctx.href`
- `ctx.path`
- `ctx.path=`
- `ctx.query`
- `ctx.query=`
- `ctx.querystring`
- `ctx.querystring=`
- `ctx.host`
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- `ctx.hostname`
- `ctx.fresh`
- `ctx.stale`
- `ctx.socket`
- `ctx.protocol`
- `ctx.secure`
- `ctx.ip`
- `ctx.ips`
- `ctx.subdomains`
- `ctx.is()`
- `ctx.accepts()`
- `ctx.acceptsEncodings()`
- `ctx.acceptsCharsets()`
- `ctx.acceptsLanguages()`
- `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=`