JavaScript Exceptions

When JavaScript hits a problem at runtime, the usual response is an exception: stop the normal flow and jump to a handler.

Creating exceptions

Use throw to raise one:

throw value

value can be a string, number, object, or an Error instance. As soon as throw runs, code after it in that path does not execute.

Handling exceptions

Wrap risky code in try/catch:

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

}

e is the thrown value. You can add multiple catch blocks in some environments, or branch on e.name inside one handler.

finally

finally runs whether or not an exception happened:

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

} finally {

}

Use finally to release resources you opened in try, such as closing a connection or clearing a flag.

You can omit catch and use try/finally when you only need cleanup:

try {
  //lines of code
} finally {

}

Nested try blocks

try blocks can nest. An exception bubbles to the nearest matching catch:

try {
  //lines of code

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

} catch (e) {

}

If the inner try has no catch, the outer catch handles the error.

Try this pattern with a function that throws when input is invalid. Log in catch, then confirm finally still runs.

Throw new Error('message') rather than a bare string when you want a useful stack trace in the console.

You can rethrow after logging if a lower layer should not swallow the failure: catch (e) { console.error(e); throw e }.

Libraries often wrap unknown thrown values with Error so callers always receive something with a stack. That keeps logs consistent across browsers.

Try this pattern with a function that throws when input is invalid. Log in catch, then confirm finally still runs.