Professional JavaScript For Web Developers

Chapter 14
Determining the type of error
Despite being limited to only one catch clause per try...catch statement, you have a couple of easy
ways to determine the type of error that was thrown. The first is to use the name property of the Error
object:
try {
eval(“a ++ b”); //causes SyntaxError
} catch (oException) {
if (oException.name == “SyntaxError”) {
alert(“Syntax Error: “ + oException.message);
} else {
alert(“An unexpected error occurred: “ + oException.message);
}
}
The second way is to use the instanceof operator and use the class name of different errors:
try {
eval(“a ++ b”); //causes SyntaxError
} catch (oException) {
if (oException instanceof SyntaxError) {
alert(“Syntax Error: “ + oException.message);
} else {
alert(“An unexpected error occurred: “ + oException.message);
}
}
Raising exceptions
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The third edition of ECMAScript also introduced the throw statement to raise exceptions purposely. The
syntax is the following:
throw error_object;
The error_object can be a string, a number, a Boolean value, or an actual object. All the following
lines are valid:
throw “An error occurred.”;
throw 50067;
throw true;
throw new Object();
It is also possible to throw an actual Error object. The constructor for the Error object takes only one
parameter, the error message, making it possible to do the following:
throw new Error(“You tried to do something bad.”);
All the other classes of Error are also available to developers:
throw new SyntaxError(“I don’t like your syntax.”);
throw new TypeError(“What type of variable do you take me for?”);
throw new RangeError(“Sorry, you just don’t have the range.”);
throw new EvalError(“That doesn’t evaluate.”);