错误处理

¥Error Handling

错误处理是指 Express 如何捕获和处理同步和异步发生的错误。Express 带有一个默认的错误处理程序，因此你无需编写自己的程序即可开始使用。

¥Error Handling refers to how Express catches and processes errors that occur both synchronously and asynchronously. Express comes with a default error handler so you don’t need to write your own to get started.

捕获错误

¥Catching Errors

确保 Express 捕获运行路由处理程序和中间件时发生的所有错误非常重要。

¥It’s important to ensure that Express catches all errors that occur while running route handlers and middleware.

路由处理程序和中间件内的同步代码中发生的错误不需要额外的工作。如果同步代码抛出错误，Express 将捕获并处理它。例如：

¥Errors that occur in synchronous code inside route handlers and middleware require no extra work. If synchronous code throws an error, then Express will catch and process it. For example:

app.get('/', (req, res) => {
  throw new Error('BROKEN') // Express will catch this on its own.
})

对于路由处理程序和中间件调用的异步函数返回的错误，你必须将它们传递给 next() 函数，Express 将在其中捕获并处理它们。例如：

¥For errors returned from asynchronous functions invoked by route handlers and middleware, you must pass them to the next() function, where Express will catch and process them. For example:

app.get('/', (req, res, next) => {
  fs.readFile('/file-does-not-exist', (err, data) => {
    if (err) {
      next(err) // Pass errors to Express.
    } else {
      res.send(data)
    }
  })
})

从 Express 5 开始，返回 Promise 的路由处理程序和中间件将在拒绝或抛出错误时自动调用 next(value)。例如：

¥Starting with Express 5, route handlers and middleware that return a Promise will call next(value) automatically when they reject or throw an error. For example:

app.get('/user/:id', async (req, res, next) => {
  const user = await getUserById(req.params.id)
  res.send(user)
})

如果 getUserById 抛出错误或拒绝，next 将使用抛出的错误或拒绝的值调用。如果没有提供拒绝值，则将使用 Express 路由提供的默认错误对象调用 next。

¥If getUserById throws an error or rejects, next will be called with either the thrown error or the rejected value. If no rejected value is provided, next will be called with a default Error object provided by the Express router.

如果你将任何内容传递给 next() 函数（字符串 'route' 除外），Express 会将当前请求视为错误，并将跳过任何剩余的非错误处理路由和中间件函数。

¥If you pass anything to the next() function (except the string 'route'), Express regards the current request as being an error and will skip any remaining non-error handling routing and middleware functions.

如果序列中的回调不提供数据，只提供错误，你可以简化此代码，如下所示：

¥If the callback in a sequence provides no data, only errors, you can simplify this code as follows:

app.get('/', [
  function (req, res, next) {
    fs.writeFile('/inaccessible-path', 'data', next)
  },
  function (req, res) {
    res.send('OK')
  }
])

在上面的例子中，next 作为 fs.writeFile 的回调提供，无论有无错误都会调用它。如果没有错误，则执行第二个处理程序，否则 Express 会捕获并处理错误。

¥In the above example, next is provided as the callback for fs.writeFile, which is called with or without errors. If there is no error, the second handler is executed, otherwise Express catches and processes the error.

你必须捕获路由处理程序或中间件调用的异步代码中发生的错误，并将它们传递给 Express 进行处理。例如：

¥You must catch errors that occur in asynchronous code invoked by route handlers or middleware and pass them to Express for processing. For example:

app.get('/', (req, res, next) => {
  setTimeout(() => {
    try {
      throw new Error('BROKEN')
    } catch (err) {
      next(err)
    }
  }, 100)
})

上面的示例使用 try...catch 块来捕获异步代码中的错误并将它们传递给 Express。如果 try...catch 块被省略，Express 将不会捕获错误，因为它不是同步处理程序代码的一部分。

¥The above example uses a try...catch block to catch errors in the asynchronous code and pass them to Express. If the try...catch block were omitted, Express would not catch the error since it is not part of the synchronous handler code.

使用 Promise 可以避免 try...catch 块的开销，或者在使用返回 Promise 的函数时。例如：

¥Use promises to avoid the overhead of the try...catch block or when using functions that return promises. For example:

app.get('/', (req, res, next) => {
  Promise.resolve().then(() => {
    throw new Error('BROKEN')
  }).catch(next) // Errors will be passed to Express.
})

由于 Promise 自动捕获同步错误和被拒绝的 Promise，你可以简单地提供 next 作为最终的 catch 处理程序，Express 将捕获错误，因为 catch 处理程序将错误作为第一个参数。

¥Since promises automatically catch both synchronous errors and rejected promises, you can simply provide next as the final catch handler and Express will catch errors, because the catch handler is given the error as the first argument.

你还可以使用一系列处理程序来依赖同步错误捕获，方法是将异步代码减少到微不足道的程度。例如：

¥You could also use a chain of handlers to rely on synchronous error catching, by reducing the asynchronous code to something trivial. For example:

app.get('/', [
  function (req, res, next) {
    fs.readFile('/maybe-valid-file', 'utf-8', (err, data) => {
      res.locals.data = data
      next(err)
    })
  },
  function (req, res) {
    res.locals.data = res.locals.data.split(',')[1]
    res.send(res.locals.data)
  }
])

上面的例子有几个来自 readFile 调用的琐碎语句。如果 readFile 导致错误，则将错误传递给 Express，否则你将在链中的下一个处理程序中快速返回同步错误处理的世界。然后，上面的示例尝试处理数据。如果失败，则同步错误处理程序将捕获它。如果你在 readFile 回调中完成了此处理，则应用可能会退出，并且 Express 错误处理程序将不会运行。

¥The above example has a couple of trivial statements from the readFile call. If readFile causes an error, then it passes the error to Express, otherwise you quickly return to the world of synchronous error handling in the next handler in the chain. Then, the example above tries to process the data. If this fails, then the synchronous error handler will catch it. If you had done this processing inside the readFile callback, then the application might exit and the Express error handlers would not run.

无论你使用哪种方法，如果你希望 Express 错误处理程序被调用并且应用能够继续存在，你必须确保 Express 接收到错误。

¥Whichever method you use, if you want Express error handlers to be called in and the application to survive, you must ensure that Express receives the error.

默认错误处理程序

¥The default error handler

Express 带有一个内置的错误处理程序，可以处理应用中可能遇到的任何错误。这个默认的错误处理中间件函数被添加到中间件函数栈的末尾。

¥Express comes with a built-in error handler that takes care of any errors that might be encountered in the app. This default error-handling middleware function is added at the end of the middleware function stack.

如果你将错误传递给 next() 而你没有在自定义错误处理程序中处理它，它将由内置错误处理程序处理；该错误将与堆栈跟踪一起写入客户端。堆栈跟踪不包含在生产环境中。

¥If you pass an error to next() and you do not handle it in a custom error handler, it will be handled by the built-in error handler; the error will be written to the client with the stack trace. The stack trace is not included in the production environment.

写入错误时，会在响应中添加以下信息：

¥When an error is written, the following information is added to the response:

• res.statusCode 由 err.status（或 err.statusCode）设置。如果此值超出 4xx 或 5xx 范围，则将设置为 500。

  ¥The res.statusCode is set from err.status (or err.statusCode). If this value is outside the 4xx or 5xx range, it will be set to 500.

• res.statusMessage 根据状态码设置。

  ¥The res.statusMessage is set according to the status code.

• 在生产环境中，body 为状态码消息的 HTML，否则为 err.stack。

  ¥The body will be the HTML of the status code message when in production environment, otherwise will be err.stack.

• err.headers 对象中指定的任何标头。

  ¥Any headers specified in an err.headers object.

如果在开始编写响应后调用 next() 并出现错误（例如，如果在将响应流式传输到客户端时遇到错误），Express 默认错误处理程序将关闭连接并使请求失败。

¥If you call next() with an error after you have started writing the response (for example, if you encounter an error while streaming the response to the client), the Express default error handler closes the connection and fails the request.

因此，当你添加自定义错误处理程序时，你必须委托默认的 Express 错误处理程序，此时标头已发送到客户端：

¥So when you add a custom error handler, you must delegate to the default Express error handler, when the headers have already been sent to the client:

function errorHandler (err, req, res, next) {
  if (res.headersSent) {
    return next(err)
  }
  res.status(500)
  res.render('error', { error: err })
}

请注意，如果你多次调用 next() 并在代码中出现错误，则可能会触发默认错误处理程序，即使自定义错误处理中间件已到位。

¥Note that the default error handler can get triggered if you call next() with an error in your code more than once, even if custom error handling middleware is in place.

其他错误处理中间件可以在 Express 中间件 找到。

¥Other error handling middleware can be found at Express middleware.

编写错误处理程序

¥Writing error handlers

以与其他中间件函数相同的方式定义错误处理中间件函数，除了错误处理函数有四个参数而不是三个：(err, req, res, next)。例如：

¥Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next). For example:

app.use((err, req, res, next) => {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

你最后定义错误处理中间件，在其他 app.use() 和路由调用之后；例如：

¥You define error-handling middleware last, after other app.use() and routes calls; for example:

const bodyParser = require('body-parser')
const methodOverride = require('method-override')

app.use(bodyParser.urlencoded({
  extended: true
}))
app.use(bodyParser.json())
app.use(methodOverride())
app.use((err, req, res, next) => {
  // logic
})

来自中间件函数的响应可以是任何格式，例如 HTML 错误页面、简单消息或 JSON 字符串。

¥Responses from within a middleware function can be in any format, such as an HTML error page, a simple message, or a JSON string.

出于组织（和更高级别的框架）的目的，你可以定义几个错误处理中间件函数，就像你使用常规中间件函数一样。例如，为使用 XHR 和不使用 XHR 发出的请求定义错误处理程序：

¥For organizational (and higher-level framework) purposes, you can define several error-handling middleware functions, much as you would with regular middleware functions. For example, to define an error-handler for requests made by using XHR and those without:

const bodyParser = require('body-parser')
const methodOverride = require('method-override')

app.use(bodyParser.urlencoded({
  extended: true
}))
app.use(bodyParser.json())
app.use(methodOverride())
app.use(logErrors)
app.use(clientErrorHandler)
app.use(errorHandler)

在此示例中，通用 logErrors 可能会将请求和错误信息写入 stderr，例如：

¥In this example, the generic logErrors might write request and error information to stderr, for example:

function logErrors (err, req, res, next) {
  console.error(err.stack)
  next(err)
}

同样在此示例中，clientErrorHandler 定义如下；在这种情况下，错误会明确传递给下一个错误。

¥Also in this example, clientErrorHandler is defined as follows; in this case, the error is explicitly passed along to the next one.

请注意，当不在错误处理函数中调用 “next” 时，你负责编写（和结束）响应。否则，这些请求将 “hang” 并且不符合垃圾收集条件。

¥Notice that when not calling “next” in an error-handling function, you are responsible for writing (and ending) the response. Otherwise, those requests will “hang” and will not be eligible for garbage collection.

function clientErrorHandler (err, req, res, next) {
  if (req.xhr) {
    res.status(500).send({ error: 'Something failed!' })
  } else {
    next(err)
  }
}

实现 “catch-all” errorHandler 函数如下（举例）：

¥Implement the “catch-all” errorHandler function as follows (for example):

function errorHandler (err, req, res, next) {
  res.status(500)
  res.render('error', { error: err })
}

如果你有一个具有多个回调函数的路由处理程序，则可以使用 route 参数跳到下一个路由处理程序。例如：

¥If you have a route handler with multiple callback functions, you can use the route parameter to skip to the next route handler. For example:

app.get('/a_route_behind_paywall',
  (req, res, next) => {
    if (!req.user.hasPaid) {
      // continue handling this request
      next('route')
    } else {
      next()
    }
  }, (req, res, next) => {
    PaidContent.find((err, doc) => {
      if (err) return next(err)
      res.json(doc)
    })
  })

在此示例中，将跳过 getPaidContent 处理程序，但 app 中用于 /a_route_behind_paywall 的任何剩余处理程序将继续执行。

¥In this example, the getPaidContent handler will be skipped but any remaining handlers in app for /a_route_behind_paywall would continue to be executed.