2.7 Error Handling

>>>

wrong

ReferenceError: Can't find variable: wrong

Errors in JavaScript are handled with exceptions. When an exception is raised, a jump is made to the nearest error handler. In this console, errors are displayed in red.

>>>

throw 'oh no!';

"oh no!"

Exceptions can also be raised manually with the throw statement.

>>>

try {
    wrong;
} catch (e) {
    print('Ooops - ' + e);
}

The try...catch statement is used to handle exceptions in code. When an exception is raised inside the try block, the code jumps to the start of the catch block.

>>>

try {
    wrong;
} finally {
    print('No matter what - this happens!');
}

ReferenceError: Can't find variable: wrong

The try statement can also have a finally block, which is always executed. Whether an exception was raised or not.

>>>

throw new Error('A decent error is a good start.');

Error: A decent error is a good start.

When throwing exceptions, care should be taken to always throw Error objects (like the example above). More on object creation and the new keyword later.

>>>

The console allows you to interact with the course material and examples. Use the following keys:

<Enter> — Execute the code
<Shift + Enter> — Add line break
<Arrow-Up> — Previous command in history
<Arrow-Down> — Next command in history

A special logging function is also available:

print(value) — Prints a value to the console.
print(value, true) — Prints a debug version of a value.

Console Learner — An interactive JavaScript course.