JavaScript Exceptions

When the code runs into an unexpected problem, the JavaScript idiomatic way to handle this situation is through exceptions.

Creating exceptions

An exception is created using the throw keyword:

throw value

where value can be any JavaScript value including a string, a number or an object.

As soon as JavaScript executes this line, the normal program flow is halted and the control is held back to the nearest exception handler.

Handling exceptions

An exception handler is a try/catch statement.

Any exception raised in the lines of code included in the try block is handled in the corresponding catch block:

try {
  //lines of code
} catch (e) {

}

e in this example is the exception value.

You can add multiple handlers, that can catch different kinds of errors.

finally

To complete this statement JavaScript has another statement called finally, which contains code that is executed regardless of the program flow, if the exception was handled or not, if there was an exception or if there wasn’t:

try {
  //lines of code
} catch (e) {

} finally {

}

You can use finally without a catch block, to serve as a way to clean up any resource you might have opened in the try block, like files or network requests:

try {
  //lines of code
} finally {

}

Nested try blocks

try blocks can be nested, and an exception is always handled in the nearest catch block:

try {
  //lines of code

  try {
    //other lines of code
  } finally {
    //other lines of code
  }

} catch (e) {

}

If an exception is raised in the inner try, it’s handled in the outer catch block.