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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
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 = 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] !== 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 }
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. */ const ZIPCODE_INVALID = -1; const 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
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; } }
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 3rd Edition (ECMA-262) | Standard | Initial definition. Implemented in JavaScript 1.4 |
ECMAScript 5.1 (ECMA-262) The definition of 'throw statement' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'throw statement' in that specification. |
Standard | |
ECMAScript 2017 Draft (ECMA-262) The definition of 'throw statement' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |