Throw statement

Overview

throw statement is used to throw exception.

throw RuntimeException("Error Message");

Expession

throw statement can have any expression but generally it should be an exception object. In general, RuntimeException will be useful for any runtime errors. As for exception objects, see try-catch-finally for details.

In Catch Clause

Only when there is throw in catch clause, no expression do not have to be written because throw statement can rethrow a caught exception.

try {
    throw RuntimeException("Some error occurred");
} catch (e) {
    throw;  // e will be re-thrown.
} finally {
    /* ... */;
}

if-modifier

throw statement can have if-modifier. See the example below.

throw RuntimeException("Some error occurred") if (a < 10);   // throw an exception if `a` is less than 10.

Examples

Example 1. Normal case with if-modifier

Code

function test(a) {
    throw RuntimeException("Some error occurred") if (a < 10);
    System.println("No throw exception because a is %{a} which means %{a} >= 10");
}
try {
    test(11);
    test(10);
    test(9);
} catch (e) {
    System.println("%{e.type()}: %{e.what()}");
}

Result

No throw exception because a is 11 which means 11 >= 10
No throw exception because a is 10 which means 10 >= 10
RuntimeException: Some error occurred

Example 2. Rethrow not in catch

Code

try {
    throw;  // what is thrown?
} catch (e) {
    System.println("%{e.type()}: %{e.what()}");
    throw;
} finally {
    System.println("Finally");
}

Result

Error: Can not use throw without expression outside catch clause near the <test.kx>:2

Example 3. Rethrow in catch

Code

try {
    throw RuntimeException("Some error occurred");
} catch (e) {
    System.println("%{e.type()}: %{e.what()}");
    throw;  // e will be re-thrown.
} finally {
    System.println("Finally");
}

Result

RuntimeException: Some error occurred
Finally
Uncaught exception: No one catch the exception.
RuntimeException: Some error occurred
Stack Trace Information:
        at <main-block>(test.kx:2)