Error Handling错误处理

Error Handling refers to how Express catches and processes errors that occur both synchronously and asynchronously. 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 comes with a default error handler so you don’t need to write your own to get started.Express附带了一个默认的错误处理程序，因此您无需编写自己的错误处理程序即可开始。

Catching Errors捕捉错误

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

Errors that occur in synchronous code inside route handlers and middleware require no extra work. 路由处理程序和中间件内部的同步代码中发生的错误不需要额外的工作。If synchronous code throws an error, then Express will catch and process it. 若同步代码抛出错误，则Express将捕获并处理该错误。For example:例如：

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

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

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

Starting with Express 5, route handlers and middleware that return a Promise will call next(value) automatically when they reject or throw an error. 从Express 5开始，返回承诺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)。For example:例如：

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

If getUserById throws an error or rejects, next will be called with either the thrown error or the rejected value. 如果getUserById抛出错误或拒绝，则将使用抛出的错误或拒绝的值调用next。If no rejected value is provided, next will be called with a default Error object provided by the Express router.如果未提供拒绝值，则将使用Express router提供的默认错误对象调用next。

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，并将跳过所有剩余的非错误处理路由和中间件函数。

If the callback in a sequence provides no data, only errors, you can simplify this code as follows:如果序列中的回调不提供数据，只提供错误，则可以按如下方式简化此代码：

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

In the above example next is provided as the callback for fs.writeFile, which is called with or without errors. 在上面的示例中，next作为fs.writeFile的回调提供，无论调用时有无错误。If there is no error the second handler is executed, otherwise Express catches and processes the error.如果没有错误，则执行第二个处理程序，否则Express将捕获并处理错误。

You must catch errors that occur in asynchronous code invoked by route handlers or middleware and pass them to Express for processing. 您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，并将其传递给Express进行处理。For example:例如：

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

The above example uses a try...catch block to catch errors in the asynchronous code and pass them to Express. 上面的示例使用try...catch块捕获异步代码中的错误并将其传递给Express。If the try...catch block were omitted, Express would not catch the error since it is not part of the synchronous handler code.如果省略了try...catch块，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。

Use promises to avoid the overhead of the try...catch block or when using functions that return promises. 使用承诺可以避免try...catch块的开销，或者在使用返回承诺的函数时。For example:例如：

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

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.由于承诺会自动捕获同步错误和被拒绝的承诺，因此您可以简单地提供next作为最终的捕获处理程序，Express将捕获错误，因为捕获处理程序将错误作为第一个参数。

You could also use a chain of handlers to rely on synchronous error catching, by reducing the asynchronous code to something trivial. 您还可以使用一系列处理程序来依赖同步错误捕获，方法是将异步代码简化为琐碎的代码。For example:例如：

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

The above example has a couple of trivial statements from the readFile call. 上面的例子有几个来自readFile调用的简单语句。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. 如果readFile导致错误，那么它会将错误传递给Express，否则您将在链中的下一个处理程序中快速返回到同步错误处理的世界。Then, the example above tries to process the data. 然后，上面的示例尝试处理数据。If this fails then the synchronous error handler will catch it. 如果失败，那么同步错误处理程序将捕获它。If you had done this processing inside the readFile callback then the application might exit and the Express error handlers would not run.如果您在readFile回调中完成了此处理，则应用程序可能会退出，而Express错误处理程序将不会运行。

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.无论使用哪种方法，如果希望调用Express错误处理程序并使应用程序继续运行，则必须确保Express接收到错误。

The default error handler默认的错误处理程序

Express comes with a built-in error handler that takes care of any errors that might be encountered in the app. Express附带一个内置的错误处理程序，用于处理应用程序中可能遇到的任何错误。This default error-handling middleware function is added at the end of the middleware function stack.此默认错误处理中间件函数添加在中间件函数堆栈的末尾。

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. 如果将错误传递给next()，而不在自定义错误处理程序中处理，则将由内置错误处理程序处理；错误将与堆栈跟踪一起写入客户端。The stack trace is not included in the production environment.生产环境中不包括堆栈跟踪。

When an error is written, the following information is added to the response:写入错误时，将向响应中添加以下信息：

• The res.statusCode is set from err.status (or err.statusCode). res.statusCode是根据err.status（或err.statusCode）设置的。If this value is outside the 4xx or 5xx range, it will be set to 500.如果该值超出4xx或5xx范围，则将其设置为500。
• The res.statusMessage is set according to the status code.res.statusMessage根据状态代码设置。
• The body will be the HTML of the status code message when in production environment, otherwise will be err.stack.在生产环境中，正文将是状态代码消息的HTML，否则将是err.stack。
• Any headers specified in an err.headers object.err.headers对象中指定的任何头。

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.如果在开始编写响应后调用next()时出错（例如，如果将响应流式传输到客户端时遇到错误），则Express default error handler将关闭连接并使请求失败。

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:因此，当添加自定义错误处理程序时，必须在标头已发送到客户端时，将其委托给默认的Express错误处理程序：

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

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.请注意，如果在代码中多次调用next()，并且出现错误，则会触发默认错误处理程序，即使已安装自定义错误处理中间件。

Writing error handlers写入错误处理程序

Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: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，但错误处理函数有四个参数，而不是三个：(err, req, res, next). For example:。例如：

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

You define error-handling middleware last, after other app.use() and routes calls; for example:最后，在其他app.use()和路由调用之后定义错误处理中间件；例如：

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

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

Responses from within a middleware function can be in any format, such as an HTML error page, a simple message, or a JSON string.中间件函数中的响应可以是任何格式，例如HTML错误页、简单消息或JSON字符串。

For organizational (and higher-level framework) purposes, you can define several error-handling middleware functions, much as you would with regular middleware functions. 出于组织（和更高级别的框架）的目的，您可以定义几个错误处理中间件功能，就像您使用常规中间件功能一样。For example, to define an error-handler for requests made by using XHR and those without:例如，要为使用XHR和不使用XHR的请求定义错误处理程序，请执行以下操作：

var bodyParser = require('body-parser')
var 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)

In this example, the generic logErrors might write request and error information to stderr, for example:在本例中，通用logErrors可能会将请求和错误信息写入stderr，例如：

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

Also in this example, clientErrorHandler is defined as follows; in this case, the error is explicitly passed along to the next one.同样在本例中，clientErrorHandler的定义如下：；在这种情况下，错误会显式传递给下一个错误。

Notice that when not calling “next” in an error-handling function, you are responsible for writing (and ending) the response. 请注意，在错误处理函数中不调用“next”时，您负责编写（并结束）响应。Otherwise those requests will “hang” and will not be eligible for garbage collection.否则，这些请求将“挂起”，并且不符合垃圾收集的条件。

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

Implement the “catch-all” errorHandler function as follows (for example):实现“捕获所有”errorHandler函数，如下所示（例如）：

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

If you have a route handler with multiple callback functions you can use the route parameter to skip to the next route handler. 如果您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。For example:例如：

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

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.在本例中，将跳过getPaidContent处理程序，但将针对/a_route_behind_paywall继续执行App后面的所有剩余处理程序。