JavaScript: The Definitive Guide, Fourth Edition

throw

An exception is a signal that indicates that some sort of exceptional condition or error has occurred. To throw an exception is to signal such an error or exceptional condition. To catch an exception is to handle it -- to take whatever actions are necessary or appropriate to recover from the exception. In JavaScript, exceptions are thrown whenever a runtime error occurs and whenever the program explicitly throws one using the throw statement. Exceptions are caught with the try/catch/finally statement, which is described in the next section.^[21]

The throw statement has the following syntax:

throw expression;

expression may evaluate to a value of any type. Commonly, however, it is an Error object or an instance of one of the subclasses of Error. It can also be useful to throw a string that contains an error message, or a numeric value that represents some sort of error code. Here is some example code that uses the throw statement to throw an exception:

function factorial(x) {
    // If the input argument is invalid, thrown an exception!
    if (x < 0) throw new Error("x must not be negative");
    // Otherwise, compute a value and return normally
    for(var f = 1; x > 1; f *= x, x--) /* empty */ ;
    return f;
}

When an exception is thrown, the JavaScript interpreter immediately stops normal program execution and jumps to the nearest exception handler. Exception handlers are written using the catch clause of the try/catch/finally statement, which is described in the next section. If the block ...