Exception handling statements

标签：

You can throw exceptions using the throw statement and handle them using thetry...catch statements.

• throw statement
• try...catch statement

Exception types

Just about any object can be thrown in JavaScript. Nevertheless, not all thrown objects are created equal. While it is fairly common to throw numbers or strings as errors it is frequently more effective to use one of the exception types specifically created for this purpose:

• ECMAScript exceptions
• DOMException and DOMError

throw statement

Use the throw statement to throw an exception. When you throw an exception, you specify the expression containing the value to be thrown:

throw expression;

You may throw any expression, not just expressions of a specific type. The following code throws several exceptions of varying types:

throw "Error2";   // String type
throw 42;         // Number type
throw true;       // Boolean type
throw {toString: function() { return "I‘m an object!"; } };

// Create an object type UserException
function UserException (message){
  this.message=message;
  this.name="UserException";
}

// Make the exception convert to a pretty string when used as a string 
// (e.g. by the error console)
UserException.prototype.toString = function () {
  return this.name + ‘: "‘ + this.message + ‘"‘;
}

// Create an instance of the object type and throw it
throw new UserException("Value too high");

try...catch statement

The try...catch statement marks a block of statements to try, and specifies one or more responses should an exception be thrown. If an exception is thrown, the try...catchstatement catches it.

The try...catch statement consists of a try block, which contains one or more statements, and zero or more catch blocks, containing statements that specify what to do if an exception is thrown in the try block. That is, you want the try block to succeed, and if it does not succeed, you want control to pass to the catch block. If any statement within the try block (or in a function called from within the try block) throws an exception, control immediately shifts to the catch block. If no exception is thrown in the try block, the catch block is skipped. The finally block executes after the try and catch blocks execute but before the statements following the try...catch statement.

The following example uses a try...catch statement. The example calls a function that retrieves a month name from an array based on the value passed to the function. If the value does not correspond to a month number (1-12), an exception is thrown with the value "InvalidMonthNo" and the statements in the catch block set the monthNamevariable to unknown.

function getMonthName (mo) {
  mo = mo-1; // Adjust month number for array index (1=Jan, 12=Dec)
  var months = ["Jan","Feb","Mar","Apr","May","Jun","Jul",
                "Aug","Sep","Oct","Nov","Dec"];
  if (months[mo] != null) {
    return months[mo];
  } else {
    throw "InvalidMonthNo"; //throw keyword is used here
  }
}

try { // statements to try
  monthName = getMonthName(myMonth); // function could throw exception
}
catch (e) {
  monthName = "unknown";
  logMyErrors(e); // pass exception object to error handler
}
 

The catch block

You can use a catch block to handle all exceptions that may be generated in the try block.

catch (catchID) {
  statements
}

The catch block specifies an identifier (catchID in the preceding syntax) that holds the value specified by the throw statement; you can use this identifier to get information about the exception that was thrown. JavaScript creates this identifier when the catchblock is entered; the identifier lasts only for the duration of the catch block; after thecatch block finishes executing, the identifier is no longer available.

For example, the following code throws an exception. When the exception occurs, control transfers to the catch block.

try {
  throw "myException" // generates an exception
}
catch (e) {
  // statements to handle any exceptions
  logMyErrors(e) // pass exception object to error handler
}

The finally block

The finally block contains statements to execute after the try and catch blocks execute but before the statements following the try...catch statement. The finally block executes whether or not an exception is thrown. If an exception is thrown, the statements in the finally block execute even if no catch block handles the exception.

You can use the finally block to make your script fail gracefully when an exception occurs; for example, you may need to release a resource that your script has tied up. The following example opens a file and then executes statements that use the file (server-side JavaScript allows you to access files). If an exception is thrown while the file is open, thefinally block closes the file before the script fails.

openMyFile();
try {
  writeMyFile(theData); //This may throw a error
} catch(e) {  
  handleError(e); // If we got a error we handle it
} finally {
  closeMyFile(); // always close the resource
}

If the finally block returns a value, this value becomes the return value of the entiretry-catch-finally production, regardless of any return statements in the try andcatch blocks:

function f() {
  try {
    console.log(0);
    throw "bogus";
  } catch(e) {
    console.log(1);
    return true; // this return statement is suspended
                 // until finally block has completed
    console.log(2); // not reachable
  } finally {
    console.log(3);
    return false; // overwrites the previous "return"
    console.log(4); // not reachable
  }
  // "return false" is executed now  
  console.log(5); // not reachable
}
f(); // alerts 0, 1, 3; returns false
 

Overwriting of return values by the finally block also applies to exceptions thrown or re-thrown inside of the catch block:

function f() {
  try {
    throw "bogus";
  } catch(e) {
    console.log(‘caught inner "bogus"‘);
    throw e; // this throw statement is suspended until 
             // finally block has completed
  } finally {
    return false; // overwrites the previous "throw"
  }
  // "return false" is executed now
}

try {
  f();
} catch(e) {
  // this is never reached because the throw inside
  // the catch is overwritten
  // by the return in finally
  console.log(‘caught outer "bogus"‘);
}

// OUTPUT
// caught inner "bogus"
 

Nesting try...catch statements

You can nest one or more try...catch statements. If an inner try...catch statement does not have a catch block, the enclosing try...catch statement‘s catch block is checked for a match.

Utilizing Error objects

Depending on the type of error, you may be able to use the ‘name‘ and ‘message‘ properties to get a more refined message. ‘name‘ provides the general class of Error (e.g., ‘DOMException‘ or ‘Error‘), while ‘message‘ generally provides a more succinct message than one would get by converting the error object to a string.

If you are throwing your own exceptions, in order to take advantage of these properties (such as if your catch block doesn‘t discriminate between your own exceptions and system ones), you can use the Error constructor. For example:

function doSomethingErrorProne () {
  if (ourCodeMakesAMistake()) {
    throw (new Error(‘The message‘));
  } else {
    doSomethingToGetAJavascriptError();
  }
}
....
try {
  doSomethingErrorProne();
}
catch (e) {
  console.log(e.name); // logs ‘Error‘
  console.log(e.message); // logs ‘The message‘ or a JavaScript error message)
}
 

Exception handling statements

标签：

原文地址：http://www.cnblogs.com/hephec/p/4601263.html