用于 Node.js 的下一代 Web 框架
当前版本 v2.15

介绍

Koa 是由 Express 背后的团队设计的一个新的 Web 框架，旨在成为 Web 应用和 API 的更小、更具表现力和更强大的基础。 通过利用异步函数，Koa 允许你放弃回调并大大提高错误处理能力。 Koa 的核心中没有捆绑任何中间件，它提供了一套优雅的方法，使编写服务器变得快速而愉快。

安装

¥Installation

Koa 需要 Node v12 或更高版本才能支持 ES2015 和异步函数。

¥Koa requires node v12 or higher for ES2015 and async function support.

你可以使用你最喜欢的版本管理器快速安装受支持的 Node 版本：

¥You can quickly install a supported version of Node with your favorite version manager:

$ nvm install 12
$ npm i koa
$ node my-koa-app.js

应用

¥Application

Koa 应用是一个包含中间件函数数组的对象，这些中间件函数根据请求以类似堆栈的方式组合和执行。Koa 与你可能遇到的许多其他中间件系统类似，例如 Ruby 的 Rack、Connect 等 - 然而，我们做出了一个关键的设计决策，即在底层中间件层提供高级别 "糖"。这提高了互操作性、健壮性，并使编写中间件变得更加愉快。

¥A Koa application is an object containing an array of middleware functions which are composed and executed in a stack-like manner upon request. Koa is similar to many other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on - however a key design decision was made to provide high level "sugar" at the otherwise low-level middleware layer. This improves interoperability, robustness, and makes writing middleware much more enjoyable.

这包括用于常见任务的方法，例如内容协商、缓存新鲜度、代理支持和重定向等。尽管提供了相当多的有用方法，Koa 仍然占用很小的空间，因为没有打包任何中间件。

¥This includes methods for common tasks like content-negotiation, cache freshness, proxy support, and redirection among others. Despite supplying a reasonably large number of helpful methods Koa maintains a small footprint, as no middleware are bundled.

惯常的 hello world 应用：

¥The obligatory hello world application:

const Koa = require('koa');
const app = new Koa();

app.use(async ctx => {
  ctx.body = 'Hello World';
});

app.listen(3000);

级联

¥Cascading

Koa 中间件以更传统的方式级联，就像你可能习惯使用类似的工具一样 - 以前，Node 使用回调很难使用户友好。然而，通过异步函数，我们可以实现 "真" 中间件。对比 Connect 的实现，它只是通过一系列函数传递控制直到一个返回，Koa 调用 "下游"，然后控制流回 "上游"。

¥Koa middleware cascade in a more traditional way as you may be used to with similar tools - this was previously difficult to make user friendly with Node's use of callbacks. However with async functions we can achieve "true" middleware. Contrasting Connect's implementation which simply passes control through series of functions until one returns, Koa invoke "downstream", then control flows back "upstream".

以下示例使用 "Hello World" 进行响应，但请求首先流经 x-response-time 和 logging 中间件以标记请求何时开始，然后通过响应中间件交出控制权。当中间件调用 next() 时，该函数会挂起并将控制权传递给定义的下一个中间件。当下游不再有中间件执行时，堆栈将展开，每个中间件将恢复执行其上游行为。

¥The following example responds with "Hello World", however first the request flows through the x-response-time and logging middleware to mark when the request started, then yields control through the response middleware. When a middleware invokes next() the function suspends and passes control to the next middleware defined. After there are no more middleware to execute downstream, the stack will unwind and each middleware is resumed to perform its upstream behaviour.

const Koa = require('koa');
const app = new Koa();

// logger

app.use(async (ctx, next) => {
  await next();
  const rt = ctx.response.get('X-Response-Time');
  console.log(`${ctx.method} ${ctx.url} - ${rt}`);
});

// x-response-time

app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  ctx.set('X-Response-Time', `${ms}ms`);
});

// response

app.use(async ctx => {
  ctx.body = 'Hello World';
});

app.listen(3000);

设置

¥Settings

应用设置是 app 实例上的属性，目前支持以下内容：

¥Application settings are properties on the app instance, currently the following are supported:

你可以将设置传递给构造函数：

¥You can pass the settings to the constructor:

const Koa = require('koa');
const app = new Koa({ proxy: true });

或动态地：

¥or dynamically:

const Koa = require('koa');
const app = new Koa();
app.proxy = true;

app.listen(...)

Koa 应用不是 HTTP 服务器的一对一表示。一个或多个 Koa 应用可以挂载在一起，以使用单个 HTTP 服务器形成更大的应用。

¥A Koa application is not a 1-to-1 representation of an HTTP server. One or more Koa applications may be mounted together to form larger applications with a single HTTP server.

创建并返回一个 HTTP 服务器，将给定参数传递给 Server#listen()。这些参数记录在 nodejs.org 上。以下是绑定到 3000 端口的无用 Koa 应用：

¥Create and return an HTTP server, passing the given arguments to Server#listen(). These arguments are documented on nodejs.org. The following is a useless Koa application bound to port 3000:

const Koa = require('koa');
const app = new Koa();
app.listen(3000);

app.listen(...) 方法只是以下内容的语法糖：

¥The app.listen(...) method is simply sugar for the following:

const http = require('http');
const Koa = require('koa');
const app = new Koa();
http.createServer(app.callback()).listen(3000);

这意味着你可以将同一个应用启动为 HTTP 和 HTTPS，或者在多个地址上启动：

¥This means you can spin up the same application as both HTTP and HTTPS or on multiple addresses:

const http = require('http');
const https = require('https');
const Koa = require('koa');
const app = new Koa();
http.createServer(app.callback()).listen(3000);
https.createServer(app.callback()).listen(3001);

app.callback()

返回适合 http.createServer() 方法处理请求的回调函数。你还可以使用此回调函数将你的 Koa 应用挂载到 Connect/Express 应用中。

¥Return a callback function suitable for the http.createServer() method to handle a request. You may also use this callback function to mount your Koa app in a Connect/Express app.

app.use(function)

将给定的中间件函数添加到该应用。app.use() 返回 this，因此是可链式的。

¥Add the given middleware function to this application. app.use() returns this, so is chainable.

app.use(someMiddleware)
app.use(someOtherMiddleware)
app.listen(3000)

是相同的

¥Is the same as

app.use(someMiddleware)
  .use(someOtherMiddleware)
  .listen(3000)

请参阅 中间件 了解更多信息。

¥See Middleware for more information.

app.keys=

设置签名的 cookie 密钥。

¥Set signed cookie keys.

这些将传递给 KeyGrip，但是你也可以传递你自己的 KeyGrip 实例。例如，以下内容是可以接受的：

¥These are passed to KeyGrip, however you may also pass your own KeyGrip instance. For example the following are acceptable:

app.keys = ['OEK5zjaAMPc3L6iK7PyUjCOziUH3rsrMKB9u8H07La1SkfwtuBoDnHaaPCkG5Brg', 'MNKeIebviQnCPo38ufHcSfw3FFv8EtnAe1xE02xkN1wkCV1B2z126U44yk2BQVK7'];
app.keys = new KeyGrip(['OEK5zjaAMPc3L6iK7PyUjCOziUH3rsrMKB9u8H07La1SkfwtuBoDnHaaPCkG5Brg', 'MNKeIebviQnCPo38ufHcSfw3FFv8EtnAe1xE02xkN1wkCV1B2z126U44yk2BQVK7'], 'sha256');

出于安全考虑，请确保密钥足够长且随机。

¥For security reasons, please ensure that the key is long enough and random.

这些密钥可以轮换，并在使用 { signed: true } 选项签署 cookie 时使用：

¥These keys may be rotated and are used when signing cookies with the { signed: true } option:

ctx.cookies.set('name', 'tobi', { signed: true });

app.context

app.context 是创建 ctx 的原型。你可以通过编辑 app.context 向 ctx 添加其他属性。这对于向 ctx 添加要在整个应用中使用的属性或方法很有用，这可能会提高性能（无中间件）和/或更容易（更少 require()），但代价是更多地依赖 ctx，这可以被视为反模式。

¥app.context is the prototype from which ctx is created. You may add additional properties to ctx by editing app.context. This is useful for adding properties or methods to ctx to be used across your entire app, which may be more performant (no middleware) and/or easier (fewer require()s) at the expense of relying more on ctx, which could be considered an anti-pattern.

例如，要从 ctx 添加对数据库的引用：

¥For example, to add a reference to your database from ctx:

app.context.db = db();

app.use(async ctx => {
  console.log(ctx.db);
});

注意：

¥Note:

错误处理

¥Error Handling

默认情况下，除非 app.silent 是 true，否则将所有错误输出到 stderr。当 err.status 是 404 或 err.expose 是 true 时，默认错误处理程序也不会输出错误。要执行自定义错误处理逻辑（例如集中式日志记录），你可以添加 "error" 事件监听器：

¥By default outputs all errors to stderr unless app.silent is true. The default error handler also won't output errors when err.status is 404 or err.expose is true. To perform custom error-handling logic such as centralized logging you can add an "error" event listener:

app.on('error', err => {
  log.error('server error', err)
});

如果 req/res 循环出现错误，无法响应客户端，也会传递 Context 实例：

¥If an error is in the req/res cycle and it is not possible to respond to the client, the Context instance is also passed:

app.on('error', (err, ctx) => {
  log.error('server error', err, ctx)
});

当发生错误并且仍然可以响应客户端时，即没有数据写入套接字时，Koa 将使用 500 "内部服务器错误" 进行适当响应。在任何一种情况下，都会触发应用级别 "error" 以用于记录目的。

¥When an error occurs and it is still possible to respond to the client, aka no data has been written to the socket, Koa will respond appropriately with a 500 "Internal Server Error". In either case an app-level "error" is emitted for logging purposes.

上下文

¥Context

Koa Context 将 Node 的 request 和 response 对象封装到一个对象中，该对象为编写 Web 应用和 API 提供了许多有用的方法。这些操作在 HTTP 服务器开发中使用得非常频繁，因此它们是在此级别添加的，而不是更高级别的框架，这将迫使中间件重新实现此通用功能。

¥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.

每个请求都会创建一个 Context，并在中间件中作为接收者或 ctx 标识符进行引用，如以下代码片段所示：

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

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

为了方便起见，许多上下文的访问器和方法只是委托给它们的 ctx.request 或 ctx.response 等效项，并且在其他方面是相同的。例如，ctx.type 和 ctx.length 委托给 response 对象，ctx.path 和 ctx.method 委托给 request。

¥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 特定方法和访问器。

¥Context specific methods and accessors.

ctx.req

节点的 request 对象。

¥Node's request object.

ctx.res

节点的 response 对象。

¥Node's response object.

不支持绕过 Koa 的响应处理。避免使用以下节点属性：

¥Bypassing Koa's response handling is not supported. Avoid using the following Node properties:

ctx.request

一个 Koa Request 对象。

¥A Koa Request object.

ctx.response

一个 Koa Response 对象。

¥A Koa Response object.

ctx.state

推荐的命名空间，用于通过中间件传递信息并将信息传递到前端视图。

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

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

ctx.app

应用实例参考。

¥Application instance reference.

ctx.app.emit

Koa 应用扩展了内部 EventEmitter。ctx.app.emit 触发一个事件，其类型由第一个参数定义。对于每个事件，你可以连接 "listeners"，这是在事件触发时调用的函数。请参阅 错误处理文档 了解更多信息。

¥Koa applications extend an internal EventEmitter. 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 for more information.

ctx.cookies.get(name, [options])

获取 cookie name 和 options：

¥Get cookie name with options:

Koa 使用 cookies 模块，其中选项被简单地传递。

¥Koa uses the cookies module where options are simply passed.

ctx.cookies.set(name, value, [options])

将 cookie name 设置为 value 和 options：

¥Set cookie name to value with options:

Koa 使用 cookies 模块，其中选项被简单地传递。

¥Koa uses the cookies module where options are simply passed.

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

Helper 方法抛出一个错误，.status 属性默认为 500，这将允许 Koa 做出适当的响应。允许以下组合：

¥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:

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

例如 ctx.throw(400, 'name required') 相当于：

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

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

请注意，这些是用户级错误，并用 err.expose 标记，这意味着这些消息适合客户端响应，但错误消息通常不是这种情况，因为你不想泄漏故障详细信息。

¥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.

你可以选择传递一个按原样合并到错误中的 properties 对象，这对于装饰向上游请求者报告的机器友好错误很有用。

¥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.

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

Koa 使用 http-errors 来创建错误。status 只能作为第一个参数传递。

¥Koa uses http-errors to create errors. status should only be passed as the first parameter.

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

Helper 方法在 !value 时抛出类似于 .throw() 的错误。类似于 Node 的 assert() 方法。

¥Helper method to throw an error similar to .throw() when !value. Similar to Node's assert() method.

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

Koa 使用 http-assert 进行断言。

¥Koa uses http-assert for assertions.

ctx.respond

要绕过 Koa 的内置响应处理，你可以显式设置 ctx.respond = false;。如果你想写入原始 res 对象而不是让 Koa 为你处理响应，请使用此选项。

¥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.

请注意，Koa 不支持使用此功能。这可能会破坏 Koa 中间件和 Koa 本身的预期功能。使用此属性被认为是一种 hack，并且只是为那些希望在 Koa 中使用传统 fn(req, res) 功能和中间件的人提供便利。

¥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.

请求别名

¥Request aliases

以下访问器和别名 请求 等效项：

¥The following accessors and alias Request equivalents:

响应别名

¥Response aliases

以下访问器和别名 响应 等效项：

¥The following accessors and alias Response equivalents:

请求

¥Request

Koa Request 对象是 Node 的普通请求对象之上的抽象，提供对日常 HTTP 服务器开发有用的附加功能。

¥A Koa Request object is an abstraction on top of Node's vanilla request object, providing additional functionality that is useful for every day HTTP server development.

API

request.header

请求标头对象。这与节点的 http.IncomingMessage 上的 headers 字段相同。

¥Request header object. This is the same as the headers field on Node's http.IncomingMessage.

request.header=

设置请求头对象。

¥Set request header object.

request.headers

请求标头对象。别名为 request.header。

¥Request header object. Alias as request.header.

request.headers=

设置请求头对象。别名为 request.header=。

¥Set request header object. Alias as request.header=.

request.method

请求方法。

¥Request method.

request.method=

设置请求方法，对于实现 methodOverride() 等中间件很有用。

¥Set request method, useful for implementing middleware such as methodOverride().

request.length

将请求内容长度返回为数字（如果存在）或 undefined。

¥Return request Content-Length as a number when present, or undefined.

request.url

获取请求 URL。

¥Get request URL.

request.url=

设置请求 URL，对于 url 重写很有用。

¥Set request URL, useful for url rewrites.

request.originalUrl

获取请求原始 URL。

¥Get request original URL.

request.origin

获取 URL 的来源，包括 protocol 和 host。

¥Get origin of URL, include protocol and host.

ctx.request.origin
// => http://example.com

request.href

获取完整的请求 URL，包括 protocol、host 和 url。

¥Get full request URL, include protocol, host and url.

ctx.request.href;
// => http://example.com/foo/bar?q=1

request.path

获取请求路径名。

¥Get request pathname.

request.path=

设置请求路径名并保留查询字符串（如果存在）。

¥Set request pathname and retain query-string when present.

request.querystring

获取不含 ? 的原始查询字符串。

¥Get raw query string void of ?.

request.querystring=

设置原始查询字符串。

¥Set raw query string.

使用 ? 获取原始查询字符串。

¥Get raw query string with the ?.

request.search=

设置原始查询字符串。

¥Set raw query string.

request.host

获取主机（主机名：端口）（如果存在）。当 app.proxy 为真时支持 X-Forwarded-Host，否则使用 Host。

¥Get host (hostname:port) when present. Supports X-Forwarded-Host when app.proxy is true, otherwise Host is used.

request.hostname

获取主机名（如果存在）。当 app.proxy 为真时支持 X-Forwarded-Host，否则使用 Host。

¥Get hostname when present. Supports X-Forwarded-Host when app.proxy is true, otherwise Host is used.

如果 host 是 IPv6，Koa 将解析委托给 WHATWG URL API，注意这可能会影响性能。

¥If host is IPv6, Koa delegates parsing to WHATWG URL API, Note This may impact performance.

request.URL

获取 WHATWG 解析的 URL 对象。

¥Get WHATWG parsed URL object.

request.type

获取请求 Content-Type 不含 "charset" 等参数。

¥Get request Content-Type void of parameters such as "charset".

const ct = ctx.request.type;
// => "image/png"

request.charset

获取请求字符集（如果存在）或 undefined：

¥Get request charset when present, or undefined:

ctx.request.charset;
// => "utf-8"

request.query

获取解析的查询字符串，当不存在查询字符串时返回空对象。请注意，此 getter 不支持嵌套解析。

¥Get parsed query-string, returning an empty object when no query-string is present. Note that this getter does not support nested parsing.

例如 "color=blue&size=small"：

¥For example "color=blue&size=small":

{
  color: 'blue',
  size: 'small'
}

request.query=

将查询字符串设置为给定对象。请注意，此设置器不支持嵌套对象。

¥Set query-string to the given object. Note that this setter does not support nested objects.

ctx.query = { next: '/login' };

request.fresh

检查请求缓存是否为 "fresh"，即内容未更改。该方法用于 If-None-Match/ETag、If-Modified-Since、Last-Modified 之间的缓存协商。应在设置其中一个或多个响应标头后引用它。

¥Check if a request cache is "fresh", aka the contents have not changed. This method is for cache negotiation between If-None-Match / ETag, and If-Modified-Since and Last-Modified. It should be referenced after setting one or more of these response headers.

// freshness check requires status 20x or 304
ctx.status = 200;
ctx.set('ETag', '123');

// cache is ok
if (ctx.fresh) {
  ctx.status = 304;
  return;
}

// cache is stale
// fetch new data
ctx.body = await db.find('something');

request.stale

request.fresh 的倒数。

¥Inverse of request.fresh.

request.protocol

返回请求协议，"https" 或 "http"。当 app.proxy 为真时支持 X-Forwarded-Proto。

¥Return request protocol, "https" or "http". Supports X-Forwarded-Proto when app.proxy is true.

request.secure

ctx.protocol == "https" 的简写，用于检查请求是否通过 TLS 发送。

¥Shorthand for ctx.protocol == "https" to check if a request was issued via TLS.

request.ip

请求远程地址。当 app.proxy 为真时支持 X-Forwarded-For。

¥Request remote address. Supports X-Forwarded-For when app.proxy is true.

request.ips

当 X-Forwarded-For 存在并且 app.proxy 启用时，将返回这些 ip 的数组，按从上游 -> 下游的顺序排列。禁用时返回空数组。

¥When X-Forwarded-For is present and app.proxy is enabled an array of these ips is returned, ordered from upstream -> downstream. When disabled an empty array is returned.

例如，如果值为 "客户端、代理 1、代理 2"，你将收到数组 ["client", "proxy1", "proxy2"]。

¥For example if the value were "client, proxy1, proxy2", you would receive the array ["client", "proxy1", "proxy2"].

大多数反向代理（nginx）通过 proxy_add_x_forwarded_for 设置 x-forwarded-for，存在一定的安全风险。恶意攻击者可以通过伪造 X-Forwarded-Forrequest 标头来伪造客户端的 IP 地址。客户端发送的请求有一个 X-Forwarded-For 的请求头为 'forged'。经过反向代理转发后，request.ips 将是 ['forged','client','proxy1','proxy2']。

¥Most of the reverse proxy(nginx) set x-forwarded-for via proxy_add_x_forwarded_for, which poses a certain security risk. A malicious attacker can forge a client's ip address by forging a X-Forwarded-Forrequest header. The request sent by the client has an X-Forwarded-For request header for 'forged'. After being forwarded by the reverse proxy, request.ips will be ['forged', 'client', 'proxy1', 'proxy2'].

Koa 提供了两种避免被绕过的选项。

¥Koa offers two options to avoid being bypassed.

如果可以控制反向代理，可以通过调整配置来避免绕过，或者使用 koa 提供的 app.proxyIpHeader 来避免读取 x-forwarded-for 来获取 ips。

¥If you can control the reverse proxy, you can avoid bypassing by adjusting the configuration, or use the app.proxyIpHeader provided by koa to avoid reading x-forwarded-for to get ips.

const app = new Koa({
  proxy: true,
  proxyIpHeader: 'X-Real-IP',
});

如果你确切知道服务器前面有多少个反向代理，你可以通过配置 app.maxIpsCount 来避免读取用户伪造的请求头：

¥If you know exactly how many reverse proxies are in front of the server, you can avoid reading the user's forged request header by configuring app.maxIpsCount:

const app = new Koa({
  proxy: true,
  maxIpsCount: 1, // only one proxy in front of the server
});

// request.header['X-Forwarded-For'] === [ '127.0.0.1', '127.0.0.2' ];
// ctx.ips === [ '127.0.0.2' ];

request.subdomains

以数组形式返回子域。

¥Return subdomains as an array.

子域是应用主域之前主机的以点分隔的部分。默认情况下，应用的域被假定为主机的最后两个部分。这可以通过设置 app.subdomainOffset 来更改。

¥Subdomains are the dot-separated parts of the host before the main domain of the app. By default, the domain of the app is assumed to be the last two parts of the host. This can be changed by setting app.subdomainOffset.

例如，如果域是 "tobi.ferrets.example.com"：如果未设置 app.subdomainOffset，则 ctx.subdomains 为 ["ferrets", "tobi"]。如果 app.subdomainOffset 为 3，则 ctx.subdomains 为 ["tobi"]。

¥For example, if the domain is "tobi.ferrets.example.com": If app.subdomainOffset is not set, ctx.subdomains is ["ferrets", "tobi"]. If app.subdomainOffset is 3, ctx.subdomains is ["tobi"].

request.is(types...)

检查传入请求是否包含 "Content-Type" 标头字段，以及是否包含任何给定的 mime type。如果没有请求体，则返回 null。如果没有内容类型，或者匹配失败，则返回 false。否则，它返回匹配的内容类型。

¥Check if the incoming request contains the "Content-Type" header field, and it contains any of the give mime types. If there is no request body, null is returned. If there is no content type, or the match fails false is returned. Otherwise, it returns the matching content-type.

// With Content-Type: text/html; charset=utf-8
ctx.is('html'); // => 'html'
ctx.is('text/html'); // => 'text/html'
ctx.is('text/*', 'text/html'); // => 'text/html'

// When Content-Type is application/json
ctx.is('json', 'urlencoded'); // => 'json'
ctx.is('application/json'); // => 'application/json'
ctx.is('html', 'application/*'); // => 'application/json'

ctx.is('html'); // => false

例如，如果你想确保仅将图片发送到给定路由：

¥For example if you want to ensure that only images are sent to a given route:

if (ctx.is('image/*')) {
  // process
} else {
  ctx.throw(415, 'images only!');
}

内容协商

¥Content Negotiation

Koa 的 request 对象包括由 accepts 和 negotiator 提供支持的有用的内容协商实用程序。这些实用程序是：

¥Koa's request object includes helpful content negotiation utilities powered by accepts and negotiator. These utilities are:

如果未提供类型，则返回所有可接受的类型。

¥If no types are supplied, all acceptable types are returned.

如果提供多种类型，将返回最佳匹配。如果未找到匹配项，则返回 false，并且你应该向客户端发送 406 "Not Acceptable" 响应。

¥If multiple types are supplied, the best match will be returned. If no matches are found, a false is returned, and you should send a 406 "Not Acceptable" response to the client.

如果缺少可接受任何类型的接受标头，则将返回第一个类型。因此，你提供的类型的顺序很重要。

¥In the case of missing accept headers where any type is acceptable, the first type will be returned. Thus, the order of types you supply is important.

request.accepts(types)

检查给定的 type(s) 是否可接受，为 true 时返回最佳匹配，否则返回 false。type 值可以是一个或多个 mime 类型字符串（例如 "application/json"）、扩展名（例如 "json"）或数组 ["json", "html", "text/plain"]。

¥Check if the given type(s) is acceptable, returning the best match when true, otherwise false. The type value may be one or more mime type string such as "application/json", the extension name such as "json", or an array ["json", "html", "text/plain"].

// Accept: text/html
ctx.accepts('html');
// => "html"

// Accept: text/*, application/json
ctx.accepts('html');
// => "html"
ctx.accepts('text/html');
// => "text/html"
ctx.accepts('json', 'text');
// => "json"
ctx.accepts('application/json');
// => "application/json"

// Accept: text/*, application/json
ctx.accepts('image/png');
ctx.accepts('png');
// => false

// Accept: text/*;q=.5, application/json
ctx.accepts(['html', 'json']);
ctx.accepts('html', 'json');
// => "json"

// No Accept header
ctx.accepts('html', 'json');
// => "html"
ctx.accepts('json', 'html');
// => "json"

你可以根据需要多次调用 ctx.accepts()，或者使用开关：

¥You may call ctx.accepts() as many times as you like, or use a switch:

switch (ctx.accepts('json', 'html', 'text')) {
  case 'json': break;
  case 'html': break;
  case 'text': break;
  default: ctx.throw(406, 'json, html, or text only');
}

request.acceptsEncodings(encodings)

检查 encodings 是否可接受，为真时返回最佳匹配，否则返回 false。请注意，你应该包含 identity 作为编码之一！

¥Check if encodings are acceptable, returning the best match when true, otherwise false. Note that you should include identity as one of the encodings!

// Accept-Encoding: gzip
ctx.acceptsEncodings('gzip', 'deflate', 'identity');
// => "gzip"

ctx.acceptsEncodings(['gzip', 'deflate', 'identity']);
// => "gzip"

当没有给出参数时，所有接受的编码都作为数组返回：

¥When no arguments are given all accepted encodings are returned as an array:

// Accept-Encoding: gzip, deflate
ctx.acceptsEncodings();
// => ["gzip", "deflate", "identity"]

请注意，如果客户端显式发送 identity;q=0，则 identity 编码（意味着无编码）可能不可接受。尽管这是一种边缘情况，但你仍然应该处理此方法返回 false 的情况。

¥Note that the identity encoding (which means no encoding) could be unacceptable if the client explicitly sends identity;q=0. Although this is an edge case, you should still handle the case where this method returns false.

request.acceptsCharsets(charsets)

检查 charsets 是否可接受，为真时返回最佳匹配，否则返回 false。

¥Check if charsets are acceptable, returning the best match when true, otherwise false.

// Accept-Charset: utf-8, iso-8859-1;q=0.2, utf-7;q=0.5
ctx.acceptsCharsets('utf-8', 'utf-7');
// => "utf-8"

ctx.acceptsCharsets(['utf-7', 'utf-8']);
// => "utf-8"

当没有给出参数时，所有接受的字符集都作为数组返回：

¥When no arguments are given all accepted charsets are returned as an array:

// Accept-Charset: utf-8, iso-8859-1;q=0.2, utf-7;q=0.5
ctx.acceptsCharsets();
// => ["utf-8", "utf-7", "iso-8859-1"]

request.acceptsLanguages(langs)

检查 langs 是否可接受，为真时返回最佳匹配，否则返回 false。

¥Check if langs are acceptable, returning the best match when true, otherwise false.

// Accept-Language: en;q=0.8, es, pt
ctx.acceptsLanguages('es', 'en');
// => "es"

ctx.acceptsLanguages(['en', 'es']);
// => "es"

当没有给出参数时，所有接受的语言都作为数组返回：

¥When no arguments are given all accepted languages are returned as an array:

// Accept-Language: en;q=0.8, es, pt
ctx.acceptsLanguages();
// => ["es", "pt", "en"]

request.idempotent

检查请求是否幂等。

¥Check if the request is idempotent.

request.socket

返回请求套接字。

¥Return the request socket.

request.get(field)

返回请求头，不区分大小写 field。

¥Return request header with case-insensitive field.

响应

¥Response

Koa Response 对象是 Node 的普通响应对象之上的抽象，提供对日常 HTTP 服务器开发有用的附加功能。

¥A Koa Response object is an abstraction on top of Node's vanilla response object, providing additional functionality that is useful for every day HTTP server development.

API

response.header

响应头对象。

¥Response header object.

response.headers

响应头对象。别名为 response.header。

¥Response header object. Alias as response.header.

response.socket

响应套接字。指向 request.socket 的 net.Socket 实例。

¥Response socket. Points to net.Socket instance as request.socket.

response.status

获取响应状态。默认情况下，response.status 设置为 404，与 Node 的 res.statusCode 默认设置为 200 不同。

¥Get response status. By default, response.status is set to 404 unlike Node's res.statusCode which defaults to 200.

response.status=

通过数字代码设置响应状态：

¥Set response status via numeric code:

注意：不要太担心记住这些字符串，如果你有拼写错误，将会抛出错误，显示此列表以便你可以进行更正。

¥NOTE: don't worry too much about memorizing these strings, if you have a typo an error will be thrown, displaying this list so you can make a correction.

由于 response.status 默认设置为 404，因此要发送不带正文且具有不同状态的响应，请按如下方式完成：

¥Since response.status default is set to 404, to send a response without a body and with a different status is to be done like this:

ctx.response.status = 200;

// Or whatever other status
ctx.response.status = 204;

response.message

获取响应状态消息。默认情况下，response.message 与 response.status 关联。

¥Get response status message. By default, response.message is associated with response.status.

response.message=

将响应状态消息设置为给定值。

¥Set response status message to the given value.

response.length=

将响应内容长度设置为给定值。

¥Set response Content-Length to the given value.

response.length

如果存在，则以数字形式返回响应内容长度，或者在可能的情况下从 ctx.body 或 undefined 中推导出来。

¥Return response Content-Length as a number when present, or deduce from ctx.body when possible, or undefined.

response.body

获取响应正文。

¥Get response body.

response.body=

将响应正文设置为以下内容之一：

¥Set response body to one of the following:

如果没有设置 response.status，Koa 会根据 response.body 自动将状态设置为 200 或 204。具体来说，如果 response.body 没有设置或者已经设置为 null 或 undefined，Koa 会自动将 response.status 设置为 204。如果你确实想发送其他状态的无内容响应，则应按以下方式覆盖 204 状态：

¥If response.status has not been set, Koa will automatically set the status to 200 or 204 depending on response.body. Specifically, if response.body has not been set or has been set as null or undefined, Koa will automatically set response.status to 204. If you really want to send no content response with other status, you should override the 204 status as the following way:

// This must be always set first before status, since null | undefined
// body automatically sets the status to 204
ctx.body = null;
// Now we override the 204 status with the desired one
ctx.status = 200;

Koa 并不防范所有可以作为响应主体的内容 - 函数没有有意义的序列化，根据你的应用返回一个布尔值可能有意义，并且虽然错误有效，但它可能不会像某些人预期的那样工作 错误的属性是不可枚举的。我们建议在你的应用中添加中间件来断言每个应用的正文类型。示例中间件可能是：

¥Koa doesn't guard against everything that could be put as a response body -- a function doesn't serialise meaningfully, returning a boolean may make sense based on your application, and while an error works, it may not work as intended as some properties of an error are not enumerable. We recommend adding middleware in your app that asserts body types per app. A sample middleware might be:

app.use(async (ctx, next) => {
  await next()

  ctx.assert.equal('object', typeof ctx.body, 500, 'some dev did something wrong')
})

字符串

¥String

Content-Type 默认为 text/html 或 text/plain，两者的默认字符集均为 utf-8。还设置了内容长度字段。

¥The Content-Type is defaulted to text/html or text/plain, both with a default charset of utf-8. The Content-Length field is also set.

缓冲

¥Buffer

Content-Type 默认为 application/octet-stream，并且还设置了 Content-Length。

¥The Content-Type is defaulted to application/octet-stream, and Content-Length is also set.

流

¥Stream

Content-Type 默认为 application/octet-stream。

¥The Content-Type is defaulted to application/octet-stream.

每当将流设置为响应正文时，.onerror 就会自动添加为 error 事件的监听器以捕获任何错误。此外，每当请求关闭（甚至过早）时，流都会被销毁。如果你不需要这两个功能，请不要直接将流设置为正文。例如，当将主体设置为代理中的 HTTP 流时，你可能不希望这样做，因为这会破坏底层连接。

¥Whenever a stream is set as the response body, .onerror is automatically added as a listener to the error event to catch any errors. In addition, whenever the request is closed (even prematurely), the stream is destroyed. If you do not want these two features, do not set the stream as the body directly. For example, you may not want this when setting the body as an HTTP stream in a proxy as it would destroy the underlying connection.

看：https://github.com/koajs/koa/pull/612 了解更多信息。

¥See: https://github.com/koajs/koa/pull/612 for more information.

下面是一个在不自动销毁流的情况下进行流错误处理的示例：

¥Here's an example of stream error handling without automatically destroying the stream:

const PassThrough = require('stream').PassThrough;

app.use(async ctx => {
  ctx.body = someHTTPStream.on('error', (err) => ctx.onerror(err)).pipe(PassThrough());
});

对象

¥Object

Content-Type 默认为 application/json。这包括普通对象 { foo: 'bar' } 和数组 ['foo', 'bar']。

¥The Content-Type is defaulted to application/json. This includes plain objects { foo: 'bar' } and arrays ['foo', 'bar'].

response.get(field)

获取响应头字段值，不区分大小写 field。

¥Get a response header field value with case-insensitive field.

const etag = ctx.response.get('ETag');

response.has(field)

如果当前在传出标头中设置了由名称标识的标头，则返回 true。标头名称匹配不区分大小写。

¥Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const rateLimited = ctx.response.has('X-RateLimit-Limit');

response.set(field, value)

将响应标头 field 设置为 value：

¥Set response header field to value:

ctx.set('Cache-Control', 'no-cache');

response.append(field, value)

附加带有值 val 的附加标头 field。

¥Append additional header field with value val.

ctx.append('Link', 'http://127.0.0.1');

response.set(fields)

用一个对象设置几个响应头 fields：

¥Set several response header fields with an object:

ctx.set({
  'Etag': '1234',
  'Last-Modified': date
});

这委托给 setHeader，它通过指定的键设置或更新标头，并且不会重置整个标头。

¥This delegates to setHeader which sets or updates headers by specified keys and doesn't reset the entire header.

response.remove(field)

删除标头 field。

¥Remove header field.

response.type

获取响应 Content-Type，不含 "charset" 等参数。

¥Get response Content-Type void of parameters such as "charset".

const ct = ctx.type;
// => "image/png"

response.type=

通过 mime 字符串或文件扩展名设置响应 Content-Type。

¥Set response Content-Type via mime string or file extension.

ctx.type = 'text/plain; charset=utf-8';
ctx.type = 'image/png';
ctx.type = '.png';
ctx.type = 'png';

注意：如果合适，会为你选择 charset，例如 response.type = 'html' 将默认为 "utf-8"。如果需要覆盖 charset，可以使用 ctx.set('Content-Type', 'text/html') 直接将响应头字段设置为值。

¥Note: when appropriate a charset is selected for you, for example response.type = 'html' will default to "utf-8". If you need to overwrite charset, use ctx.set('Content-Type', 'text/html') to set response header field to value directly.

response.is(types...)

与 ctx.request.is() 非常相似。检查响应类型是否是提供的类型之一。这对于创建操纵响应的中间件特别有用。

¥Very similar to ctx.request.is(). Check whether the response type is one of the supplied types. This is particularly useful for creating middleware that manipulate responses.

例如，这是一个缩小除流之外的所有 HTML 响应的中间件。

¥For example, this is a middleware that minifies all HTML responses except for streams.

const minify = require('html-minifier');

app.use(async (ctx, next) => {
  await next();

  if (!ctx.response.is('html')) return;

  let body = ctx.body;
  if (!body || body.pipe) return;

  if (Buffer.isBuffer(body)) body = body.toString();
  ctx.body = minify(body);
});

response.redirect(url, [alt])

执行 [302] 重定向到 url。

¥Perform a [302] redirect to url.

字符串 "back" 是特殊大小写的，用于在引用者不存在时提供引用者支持，而使用 alt 或 "/"。

¥The string "back" is special-cased to provide Referrer support, when Referrer is not present alt or "/" is used.

ctx.redirect('back');
ctx.redirect('back', '/index.html');
ctx.redirect('/login');
ctx.redirect('http://google.com');

要更改 302 的默认状态，只需在调用之前或之后指定状态即可。要更改主体，请在此调用后分配它：

¥To alter the default status of 302, simply assign the status before or after this call. To alter the body, assign it after this call:

ctx.status = 301;
ctx.redirect('/cart');
ctx.body = 'Redirecting to shopping cart';

response.attachment([filename], [options])

将 Content-Disposition 设置为 "attachment" 以指示客户端提示下载。可以选择指定下载的 filename 和一些 options。

¥Set Content-Disposition to "attachment" to signal the client to prompt for download. Optionally specify the filename of the download and some options.

response.headerSent

检查响应标头是否已发送。对于查看是否可以通知客户端错误很有用。

¥Check if a response header has already been sent. Useful for seeing if the client may be notified on error.

response.lastModified

将 Last-Modified 标头作为 Date 返回（如果存在）。

¥Return the Last-Modified header as a Date, if it exists.

response.lastModified=

将 Last-Modified 标头设置为适当的 UTC 字符串。你可以将其设置为 Date 或日期字符串。

¥Set the Last-Modified header as an appropriate UTC string. You can either set it as a Date or date string.

ctx.response.lastModified = new Date();

response.etag=

设置包含封装的 " 的响应的 ETag。请注意，没有相应的 response.etag getter。

¥Set the ETag of a response including the wrapped "s. Note that there is no corresponding response.etag getter.

ctx.response.etag = crypto.createHash('md5').update(ctx.body).digest('hex');

response.vary(field)

随 field 变化。

¥Vary on field.

response.flushHeaders()

刷新所有设置的标头，然后开始正文。

¥Flush any set headers, and begin the body.

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