Error Handling

The throw Statement

The throw statement allows you to throw an exception:

throw 'Something bad happened';

You can theoretically throw any value. However, you will usually throw Error objects:

throw new Error('Something bad happened');

Remember that the new operator creates a new object (of some class).

The try...catch Statement

The try...catch statement specifies a block of statements to execute "normally" and a block to execute if the "normal" block throws an exception. The normal block is contained in a try block while the "exception" block is contained in a catch block. If any statement in the try block throws an exception, code execution jumps to the catch block immediately—the rest of the try block is ignored:

function divide(x, y) {
  if (y === 0) {
    throw new Error('Division by 0 is a bad idea');
  }

  return x / y;
}

try {
  console.log('try block');
  console.log(divide(3, 0));
} catch (e) {
  console.log('catch block');
}

This will log:

try block
catch block

Note that if the try block doesn't throw an exception, the catch block is never executed.

Consider this example:

function divide(x, y) {
  if (y === 0) {
    throw new Error('Division by 0 is a bad idea');
  }

  return x / y;
}

try {
  console.log('try block');
  console.log(divide(3, 1));
} catch (e) {
  console.log('catch block');
}

This will log:

try block
3

The finally Block

The finally block is an optional block that can be used after catch if some cleanup code should always run after the try block:

function divide(x, y) {
  if (y === 0) {
    throw new Error('Division by 0 is a bad idea');
  }

  return x / y;
}

try {
  console.log('try block');
  console.log(divide(3, 0));
} catch (e) {
  console.log('catch block');
} finally {
  console.log('finally block');
}

This will output:

try block
catch block
finally block

The finally block will become useful once we write more complex code that performs resource management and we need to perform cleanup of a resource regardless of whether an exception is thrown or not.

Error Objects

Error objects have a name and a message property which can be used to gather information when an error is thrown:

function divide(x, y) {
  if (y === 0) {
    throw new Error('Division by 0 is a bad idea');
  }
}

try {
  console.log('try block');
  divide(3, 0);
} catch (e) {
  console.log('catch block');
  console.log(e.name);
  console.log(e.message);
}

This will log:

try block
catch block
Error
Division by 0 is a bad idea