CIS 119 Handling exceptions

Objectives

Use try, throw, and catch
Use console.assert() to catch errors

Try, throw, catch

You can check for exceptions using the "try/catch" structure.
You can throw an exception in code using the "throw" keyword.
If an exception is thrown in a function and is not inside a "try" block, then the rest of the code in the function does not execute.
Any code that might cause an exception should be in a "try" code block.
A "catch" code block follows the "try" block.
If an exception is encountered in the "try" block, execution immediately jumps to the "catch" block.
Keep in mind that any code in the "try" block following an exception is not executed.
An optional "finally" block can follow the "catch" block. The "finally" block code will run whether there was an error or not.

Exception examples

// example of code that throws exceptions
function add(a, b) {
    if (arguments.length != 2) {
        throw "Invalid number of arguments";
    }
    if (typeof a != 'number' || typeof b != 'number') {
        throw "Illegal argument exception";
    }
    if (!isFinite(a) || !isFinite(b)) {
        throw "Illegal number exception";
    }
    return a + b;
}

// this will display: Invalid number of arguments
try { 
    console.log(add());
} catch (err) {
    console.log(err);
}

// this will display: Illegal argument exception
try { 
    console.log(add("1", "2"));
} catch (err) {
    console.log(err);
}

// this will display: 7
try { 
    console.log(add(3, 4));
} catch (err) {
    console.log(err);
}

// this will display: Invalid number of arguments
try { 
    console.log(add(1,2,3));
} catch (err) {
    console.log(err);
}

// this will display: Illegal number exception
try {  
    console.log(add(5/0,3));
} catch (err) {
    console.log(err);
}

// this will display: Illegal argument exception
try { 
    console.log(add(5,"2"));
} catch (err) {
    console.log(err);
}

Using assert

Note: console.assert should work well on Chrome, but may not be supported (or supported well) in other browsers
console.assert() is used for catching internal logic errors
console.assert() should not be used for validating user input
console.assert(expression): will cause an error message to print on the console if expression evaluates to false
console.assert(expression, message): will cause an error message to print on the console if expression evaluates to false; the message will be included with the error message

Assertion examples

// example of code that throws exceptions
function add(a, b) {
    console.assert(arguments.length==2, "Invalid number of arguments");
    console.assert(typeof a == 'number' && typeof b == 'number', "Illegal argument exception");
    console.assert(isFinite(a) && isFinite(b), "Illegal number exception");
    return a + b;
}

console.log(add());          // displays: Invalid number of arguments
console.log(add("1", "2"));  // displays: Illegal argument exception
console.log(add(3, 4));      // displays: 7
console.log(add(1,2,3));     // displays: Invalid number of arguments
console.log(add(5/0,3));     // displays: Illegal number exception
console.log(add(5,"2"));     // displays: Illegal argument exception