throw

 

Summary

Throws a user-defined exception.

Version Information

Statement
Implemented in:	JavaScript 1.4
ECMA Version:	ECMA-262, Edition 3

Syntax

throw expression;

Parameters

expression

The expression to throw.

Description

Use the throw statement to throw an exception. When you throw an exception, expression specifies the value of the exception. Each of the following throws an exception:

• throw "Error2"; // generates an exception with a string value
• throw 42; // generates an exception with the value 42
• throw true; // generates an exception with the value true

Examples

Example: Throw an object

You can specify an object when you throw an exception. You can then reference the object's properties in the catch block. The following example creates an object myUserException of type UserException and uses it in a throw statement.

function UserException(message) {
   this.message = message;
   this.name = "UserException";
}
function getMonthName(mo) {
   mo = mo-1; // Adjust month number for array index (1=Jan, 12=Dec)
   var months = new Array("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul",
      "Aug", "Sep", "Oct", "Nov", "Dec");
   if (months[mo] !== undefined) {
      return months[mo];
   } else {
      throw new UserException("InvalidMonthNo");
   }
}

try {
   // statements to try
   var myMonth = 15; // 15 is out of bound to raise the exception
   monthName = getMonthName(myMonth);
} catch (e) {
   monthName = "unknown";
   logMyErrors(e.message, e.name); // pass exception object to err handler
}

Example: Another example of throwing an object

The following example tests an input string for a U.S. zip code. If the zip code uses an invalid format, the throw statement throws an exception by creating an object of type ZipCodeFormatException.

/*
 * Creates a ZipCode object.
 *
 * Accepted formats for a zip code are:
 *    12345
 *    12345-6789
 *    123456789
 *    12345 6789
 *
 * If the argument passed to the ZipCode constructor does not
 * conform to one of these patterns, an exception is thrown.
 */

function ZipCode(zip) {
   zip = new String(zip);
   pattern = /[0-9]{5}([- ]?[0-9]{4})?/;
   if (pattern.test(zip)) {
      // zip code value will be the first match in the string
      this.value = zip.match(pattern)[0];
      this.valueOf = function() {
         return this.value
      };
      this.toString = function() {
         return String(this.value)
      };
   } else {
      throw new ZipCodeFormatException(zip);
   }
}

function ZipCodeFormatException(value) {
   this.value = value;
   this.message = "does not conform to the expected format for a zip code";
   this.toString = function() {
      return this.value + this.message
   };
}

/*
 * This could be in a script that validates address data
 * for US addresses.
 */

var ZIPCODE_INVALID = -1;
var ZIPCODE_UNKNOWN_ERROR = -2;

function verifyZipCode(z) {
   try {
      z = new ZipCode(z);
   } catch (e) {
      if (e instanceof ZipCodeFormatException) {
         return ZIPCODE_INVALID;
      } else {
         return ZIPCODE_UNKNOWN_ERROR;
      }
   }
   return z;
}

a = verifyZipCode(95060);         // returns 95060
b = verifyZipCode(9560;)          // returns -1
c = verifyZipCode("a");           // returns -1
d = verifyZipCode("95060");       // returns 95060
e = verifyZipCode("95060 1234");  // returns 95060 1234

Example: Rethrow an exception

You can use throw to rethrow an exception after you catch it. The following example catches an exception with a numeric value and rethrows it if the value is over 50. The rethrown exception propagates up to the enclosing function or to the top level so that the user sees it.

try {
   throw n; // throws an exception with a numeric value
} catch (e) {
   if (e <= 50) {
      // statements to handle exceptions 1-50
   } else {
      // cannot handle this exception, so rethrow
      throw e;
   }
}

See also

try...catch