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throw Keyword

Site: Saylor Academy
Course: PRDV401: Introduction to JavaScript I
Book: throw Keyword
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Date: Tuesday, 13 May 2025, 9:14 PM

Table of contents

throw Keyword

The throw statement throws a user-defined exception. Execution of the current function will stop (the statements after throw won't be executed), and control will be passed to the first catch block in the call stack. If no catch block exists among caller functions, the program will terminate.

Syntax

throw expression;

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
throw new Error('Required');  // generates an error object with the message of Required

Also note that the throw statement is affected by automatic semicolon insertion (ASI) as no line terminator between the throw keyword and the expression is allowed.


Examples

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 of type UserException and uses it in a throw statement. 

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

let monthName;

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


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

class ZipCodeFormatException extends Error {
  constructor(zip) {
    super(`${zip} does not conform to the expected format for a zip code`);
  }
}

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

const ZIPCODE_INVALID = -1;
const ZIPCODE_UNKNOWN_ERROR = -2;

function verifyZipCode(z) {
  try {
    z = new ZipCode(z);
  } catch (e) {
    const isInvalidCode = e instanceof ZipCodeFormatException;
    return isInvalidCode ? ZIPCODE_INVALID : 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


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;
  }
}