chapter4-ErrorHandling.html

<html>
<body>
<script type="text/javascript">

// Error Handling

//Returning a special value
function between(string, start, end) {
    var startAt = string.indexOf(start);
    if (startAt == -1)
        return undefined;
    startAt += start.length;
    var endAt = string.indexOf(end, startAt);
    if(EndAt == -1)
        return undefined;
        
    return string.slice(startAt,endAt);
    
}

var input = promprt("Tell me something: ");
var parenthesized = between(input, "(", ")",);
if (parenthesized != undefined)
    print("You parenthesized '", parenthesized, "' .");
    

function lastElement(array) {
    if (array.length > 0)
        return array[array.length - 1];
    else
        return undefined;
}

lastElement([1,2,undefined]);
// was there a last element? Undefined is returned, so you don't know

// Exceptions
function lastElement(array) {
    if (array.length > 0)
        return array[array.length - 1];
    else
        throw "Cannot take the last element of an empty array.";
}

function lastElementPlusTen(array) {
    return lastElement(array) +10;
}

try {
    print(lastElementPlusTen([]));
}
catch(error) {
    print("Something went wrong: ", error);
}
 
//Clean up after exceptions with the "finally" block
function processThing(thing) {
    var prevThing = currentThing;
    currentThing = thing;
    try {
        // do complicated thing that might throw exception
    }
    finally {
        currentThing=prevThing;
    }
}

// Error objects
//as seen earlier, the "error" can be thrown
// or you can manually throw one
throw new Error("Wolf!");

//Selective Catching

var InvalidInputError = new Error("Invalid Numeric Input");

function inputNumber() {
    var input = Number(prompt("Give me a number: ", ""));
    if (isNaN(input))
        throw InvalidInputError;
    return input;
}

try {
    alert(inputNumber() + 5);
    break;
}
catch(e) {
    if(e != InvalidInputError)
        throw e;
    alert("You did not input a number. Try again.");
}


</script>
</body>
</html>