Question

I have the following code that works without using throw however when I use the throw keyword it doesn’t return the appropriate message. What am I doing wrong?

Update

I am deliberately calling the function addme as opposed to addMe as I want to catch the error message.

Code – That works without using throw

function addMe() {
        var a = 1;
        var b = 2;
        return a+b;
    }

    try {
        addme();
    }


    catch (err) {
        document.write(err.name + " " + err.message);
    }

Code – That does not work

function addMe() {
        var a = 1;
        var b = 2;
        return a+b;
    }

    try {
        addme();
        throw "error 1";
    }

    catch(err) {
        if(err ==  "error 1") {
            document.write("This is a custom message for error 1");
        }
    }

Answer 1

addme is undefined, so you never reach your throw statement. (Specifically, a ReferenceError is thrown first when you call addme rather than addMe)

The key thing to remember is the program is read from the top down – until you inject a GOTO (an error, calling a function, returning from a function, etc.) that sends you somewhere else. Once you GOTO Raptor the lines beneath that line are not guaranteed to be called.

try {
    addme(); // Reference Error Thrown - go to catch statement
    throw "error 1"; // We never get here
}

catch(err) {
    // Never true - err is always a ReferenceError.
    if(err ==  "error 1") {
        document.write("This is a custom message for error 1");
    }

To deal with any type of error you can check err instanceof TYPE_OF_ERROR:

catch(err) {
    if(err instanceof ReferenceError) {
        document.write("This is a custom message for ReferenceErrors");
    }
}

(And please remember, a string is not an error)