Coffeescript 简明教程

CoffeeScript - Exception Handling

当一个程序正在运行时，一个异常（或异常事件）是一个突发问题。当一个异常发生时，程序的正常流程被打断，并且程序/应用程序异常终止，这是不推荐的，因此这些异常应被处理。

An exception (or exceptional event) is a problem that arises during the execution of a program. When an Exception occurs, the normal flow of the program is disrupted and the program/Application terminates abnormally, which is not recommended, therefore these exceptions are to be handled.

一个异常可能由于许多不同的原因发生。以下是一些异常发生的情况。

An exception can occur for many different reasons. Here are some scenarios where an exception occurs.

  1. A user has entered invalid data.

  2. A file that needs to be opened cannot be found.

Exceptions in CoffeeScript

CoffeeScripts使用 try catch and finally 块支持异常/错误处理。这些块的功能与JavaScript相同， try 块保存异常声明， catch 块在异常发生时需要执行的操作， finally 块用于无条件地执行语句。

CoffeeScripts supports exception/error handling using the try catch and finally blocks. The functionalities of these blocks are same as in JavaScript, the try block holds the exceptional statements, the catch block has the action to be performed when an exception occurs, and the finally block is used to execute the statements unconditionally.

以下是CoffeeScript中 try catch 和 finally 块的语法。

Following are the syntaxes of try catch and finally blocks in CoffeeScript.

 try
   // Code to run

 catch ( e )
   // Code to run if an exception occurs

 finally
   // Code that is always executed regardless of
   // an exception occurring

try 块必须紧跟一个 catch 块或一个 finally 块（或者两者）。当在 try 块中发生异常时，该异常被置入 e 中，且 catch 块被执行。可选的 finally 块会在 try/catch 之后无条件执行。

The try block must be followed by either exactly one catch block or one finally block (or one of both). When an exception occurs in the try block, the exception is placed in e and the catch block is executed. The optional finally block executes unconditionally after try/catch.

Example

以下示例演示了使用CoffeeScript中的try和catch块进行异常处理。在这里，我们试图在CoffeeScript操作中使用一个未定义的符号，我们使用 try 和 catch 块处理了发生的错误。使用 Exception_handling.coffee 名称将此代码保存在一个文件中

The following example demonstrates the Exception handling using try and catch blocks in CoffeeScript. In here, we are trying to use an undefined symbol in CoffeeScript operation and we handled the error occurred using the try and catch blocks. Save this code in a file with the name Exception_handling.coffee

try
  x = y+20
  console.log "The value of x is :" +x
catch e
  console.log "exception/error occurred"
  console.log "The STACKTRACE for the exception/error occurred is ::"
  console.log e.stack

打开 command prompt 并按照以下所示编译 .coffee 文件。

Open the command prompt and compile the .coffee file as shown below.

c:\> coffee -c Exception_handling.coffee

编译后，它会给你以下 JavaScript。

On compiling, it gives you the following JavaScript.

// Generated by CoffeeScript 1.10.0
(function() {
  var e, error, x;

  try {
    x = y + 20;
    console.log("The value of x is :" + x);
  } catch (error) {
    e = error;
    console.log("exception/error occurred");
    console.log("The STACKTRACE for the exception/error occurred is ::");
    console.log(e.stack);
  }

}).call(this);

现在，再次打开 command prompt 并按照以下所示运行 CoffeeScript 文件。

Now, open the command prompt again and run the CoffeeScript file as shown below.

c:\> coffee Exception_handling.coffee

执行后，CoffeeScript 文件产生以下输出。

On executing, the CoffeeScript file produces the following output.

exception/error occurred
The STACKTRACE for the exception/error occurred is ::
ReferenceError: y is not defined
  at Object.<anonymous> (C:\Examples\strings_exceptions\Exception_handling.coffee:3:7)
  at Object.<anonymous> (C:\Examples\strings_exceptions\Exception_handling.coffee:2:1)
  at Module._compile (module.js:413:34)
  at Object.exports.run (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\coffee-script.js:134:23)
  at compileScript (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:224:29)
  at compilePath (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:174:14)
  at Object.exports.run (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:98:20)
  at Object.<anonymous> (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\bin\coffee:7:41)
  at Module._compile (module.js:413:34)
  at Object.Module._extensions..js (module.js:422:10)
  at Module.load (module.js:357:32)
  at Function.Module._load (module.js:314:12)
  at Function.Module.runMain (module.js:447:10)
  at startup (node.js:139:18)
  at node.js:999:3

The finally block

我们也可以使用 finally 块重写上面的示例。如果这样做，此块的内容将在 try 和 catch 之后无条件地执行。使用 Exception_handling_finally.coffee 名称将此代码保存在文件中

We can also rewrite the above example using finally block. If we do so, the contents of this block are executed unconditionally after try and catch. Save this code in a file with the name Exception_handling_finally.coffee

try
  x = y+20
  console.log "The value of x is :" +x
catch e
  console.log "exception/error occurred"
  console.log "The STACKTRACE for the exception/error occurred is ::"
  console.log e.stack

finally
  console.log "This is the statement of finally block"

打开 command prompt 并按照以下所示编译 .coffee 文件。

Open the command prompt and compile the .coffee file as shown below.

c:\> coffee -c Exception_handling_finally.coffee

编译后，它会给你以下 JavaScript。

On compiling, it gives you the following JavaScript.

// Generated by CoffeeScript 1.10.0
(function() {
  var e, error, x;

  try {
    x = y + 20;
    console.log("The value of x is :" + x);
  } catch (error) {
    e = error;
    console.log("exception/error occurred");
    console.log("The STACKTRACE for the exception/error occurred is ::");
    console.log(e.stack);
  } finally {
    console.log("This is the statement of finally block");
  }

}).call(this);

现在，再次打开 command prompt 并按照以下所示运行 CoffeeScript 文件。

Now, open the command prompt again and run the CoffeeScript file as shown below.

c:\> coffee Exception_handling_finally.coffee

执行后，CoffeeScript 文件产生以下输出。

On executing, the CoffeeScript file produces the following output.

exception/error occurred
The STACKTRACE for the exception/error occurred is ::
ReferenceError: y is not defined
  at Object.<anonymous> (C:\Examples\strings_exceptions\Exception_handling.coffee:3:7)
  at Object.<anonymous> (C:\Examples\strings_exceptions\Exception_handling.coffee:2:1)
  at Module._compile (module.js:413:34)
  at Object.exports.run (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\coffee-script.js:134:23)
  at compileScript (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:224:29)
  at compilePath (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:174:14)
  at Object.exports.run (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\lib\coffee-script\command.js:98:20)
  at Object.<anonymous> (C:\Users\Tutorialspoint\AppData\Roaming\npm\node_modules\coffee-script\bin\coffee:7:41)
  at Module._compile (module.js:413:34)
  at Object.Module._extensions..js (module.js:422:10)
  at Module.load (module.js:357:32)
  at Function.Module._load (module.js:314:12)
  at Function.Module.runMain (module.js:447:10)
  at startup (node.js:139:18)
  at node.js:999:3

This is the statement of finally block

The throw Statement

CoffeeScript还支持 throw 语句。您可以使用throw语句引发您的内置异常或自定义异常。稍后可以捕获这些异常，您可以采取适当的措施。

CoffeeScript also supports the throw statement. You can use throw statement to raise your builtin exceptions or your customized exceptions. Later these exceptions can be captured and you can take an appropriate action.

Example

以下示例演示了在CoffeeScript中使用 throw 语句。使用名称 throw_example.coffee 将此代码保存在文件中

The following example demonstrates the usage of the throw statement in CoffeeScript. Save this code in a file with name throw_example.coffee

myFunc = ->
  a = 100
  b = 0
  try
    if b == 0
      throw ("Divided by zero error.")
    else
      c = a / b
  catch e
    console.log "Error: " + e

myFunc()

打开 command prompt 并按照以下所示编译 .coffee 文件。

Open the command prompt and compile the .coffee file as shown below.

c:\> coffee -c throw_example.coffee

编译后，它会给你以下 JavaScript。

On compiling, it gives you the following JavaScript.

// Generated by CoffeeScript 1.10.0
(function() {
  var myFunc;

  myFunc = function() {
    var a, b, c, e, error;
    a = 100;
    b = 0;
    try {
      if (b === 0) {
        throw "Divided by zero error.";
      } else {
        return c = a / b;
      }
    } catch (error) {
      e = error;
      return console.log("Error: " + e);
    }
  };

  myFunc();

}).call(this);

现在，再次打开 command prompt 并按照以下所示运行 CoffeeScript 文件。

Now, open the command prompt again and run the CoffeeScript file as shown below.

c:\> coffee throw_example.coffee

执行后，CoffeeScript 文件产生以下输出。

On executing, the CoffeeScript file produces the following output.

Divided by zero error.