Node中的异常处理

异常处理是指在应用程序运行时处理代码中发生的异常的机制。 Node.js 支持多种传播和处理错误的机制。
这是可用于 Node.js 中异常处理的不同方法：

• 同步代码中的异常处理：如果同步代码发生错误，则返回错误。
  例子：

  // Write Javascript code here
  // Define divider as a syncrhonous function
  var divideSync = function(x, y) {
      // if error condition?
      if ( y === 0 ) {
          // "throw" the error safely by returning it
          return new Error("Can't divide by zero")
      }
      else {
          // no error occurred, continue on
          return x/y
      }
  }
    
  // Divide 9/3
  var result = divideSync(9, 3)
  // did an error occur?
  if ( result instanceof Error ) {
      // handle the error safely
      console.log("9/3=err", result)
  }
  else {
      // no error occurred, continue on
      console.log("9/3="+result)
  }
    
  // Divide 9/0
  result = divideSync(9, 0)
  // did an error occur?
  if ( result instanceof Error ) {
      // handle the error safely
      console.log("9/0=err", result)
  }
  else {
      // no error occurred, continue on
      console.log("9/0="+result)
  }
  

  输出：
  

• 基于回调（异步）代码中的异常处理：在基于回调的代码中，回调的参数之一是 err。如果发生错误，则 err 为错误，如果未发生错误，则 err 为空。 err 参数后面可以跟任意数量的其他参数。
  例子：

  // Write Javascript code here
  var divide = function(x, y, next) {
      // if error condition?
      if ( y === 0 ) {
          // "throw" the error safely by calling the completion callback
          // with the first argument being the error
          next(new Error("Can't divide by zero"))
      }
      else {
          // no error occurred, continue on
          next(null, x/y)
      }
  }
    
  divide(9, 3, function(err, result){
      // did an error occur?
      if ( err ) {
          // handle the error safely
          console.log("9/3=err", err)
      }
      else {
          // no error occurred, continue on
          console.log("9/3="+result)
      }
  })
    
  divide(9, 0, function(err, result){
      // did an error occur?
      if ( err ) {
          // handle the error safely
          console.log("9/0=err", err)
      }
      else {
          // no error occurred, continue on
          console.log("9/0="+result)
      }
  })
  

  输出：
  

• 事件代码中的异常处理：在事件代码中，错误可能发生在任何地方。因此，与其抛出错误，不如触发错误事件。
  例子：

  // Write Javascript code here
  // Definite our Divider Event Emitter
  var events = require("events")
  var Divider = function(){
      events.EventEmitter.call(this)
  }
  require('util').inherits(Divider, events.EventEmitter)
    
  // Add the divide function
  Divider.prototype.divide = function(x, y){
      // if error condition?
      if ( y === 0 ) {
          // "throw" the error safely by emitting it
          var err = new Error("Can't divide by zero")
          this.emit("error", err)
      }
      else {
          // no error occurred, continue on
          this.emit("divided", x, y, x/y)
      }
    
      // Chain
      return this;
  }
    
  // Create our divider and listen for errors
  var divider = new Divider()
  divider.on('error', function(err){
      // handle the error safely
      console.log(err)
  })
  divider.on('divided', function(x, y, result){
      console.log(x+"/"+y+"="+result)
  })
    
  // Divide
  divider.divide(9, 3).divide(9, 0)
  

  输出：