[Edit]

JavaScript - try / catch statement

1 contributors

2 contributions

0 discussions

3 points

Created by:

Palpys

764

In this article, we would like to show you how to use try / catch statement in JavaScript.

Introduction

Edit

There are four basic keywords related to the topic that you need to know:

try - is used to test the code wrapped with curly braces ({}) for errors,
catch - is executed if an exception is thrown in the try block,
finally - executes the code wrapped with curly braces after try... catch block regardless of the result,
throw - is used to create and throw custom errors.

try...catch...finally cases

Edit

Syntax / example Description

Syntax / example	Description
xxxxxxxxxx 1 try { 2 // place for the code where exception 3 // can be thrown in the safe way 4 } catch(error) { 5 // this block is executed only if exception occurs 6 }	This solution catches exceptions that appear in the code in the `try` block. If the exception occurs, the `catch` is executed.
xxxxxxxxxx 1 try { 2 // place for the code where exception 3 // can be thrown in the safe way 4 } catch(error) { 5 // this block is executed only if exception occurs 6 } finally { 7 // this block is executed always 8 // after try and catch blocks 9 // (even when exception occurs) 10 }	The same scenario as in the example above, with one difference - regardless of the exception being thrown (or not), the `finally` block will always be executed after the `try` and `catch` blocks.
xxxxxxxxxx 1 try { 2 // place for the code where exception 3 // can be thrown in the safe way 4 } finally { 5 // this block is executed always 6 // after try and catch blocks 7 // (even when exception occurs) 8 }	In this case, the exception is not caught by the `catch` block but forwarded. Finally block always executes after a `try` block.

xxxxxxxxxx
 
try {
  // place for the code where exception
  // can be thrown in the safe way
} catch(error) {
  // this block is executed only if exception occurs
}

This solution catches exceptions that appear in the code in the try block. If the exception occurs, the catch is executed.

xxxxxxxxxx
 
try {
  // place for the code where exception
  // can be thrown in the safe way
} catch(error) {
  // this block is executed only if exception occurs
} finally {
  // this block is executed always 
  // after try and catch blocks 
  // (even when exception occurs)
}

The same scenario as in the example above, with one difference - regardless of the exception being thrown (or not), the finally block will always be executed after the try and catch blocks.

xxxxxxxxxx
 
try {
  // place for the code where exception
  // can be thrown in the safe way
} finally {
  // this block is executed always 
  // after try and catch blocks 
  // (even when exception occurs)
}

In this case, the exception is not caught by the catch block but forwarded.

Finally block always executes after a try block.

Practical examples

Edit

Try... catch block

Edit

In this example, we present how to create a simple try block that throws an error and how to handle the error with the catch block.

Practical example:

xxxxxxxxxx
 
try {
  let x = y;
} catch (error) {
  console.error(error); // ReferenceError: y is not defined
}

Auto running

Throw statement

Edit

In this section, we present how to throw a custom error (throw exception) using throw statement.

xxxxxxxxxx
 
try {
  let x = 3;
  if (x != 0) {
    throw 'Custom error message.'
  }
  console.log('Try block executed.');
} catch (error) {
  console.error(error); // Custom error message.
}

Auto running

Note:

You can throw different types of exceptions.

Finally statement

Edit

In this section, we present three different cases of finally block execution.

1. When exception is thrown you can see both messages in the console.

xxxxxxxxxx
 
try {
  const x = 3;
  if (x != 0) {
    throw 'Custom error message.'
  }
  console.log('Try block executed.');
} catch (error) {
  console.error(error);
} finally {
  console.log('Finally block message (executed on success and exception).');
}

Auto running

2. When exception isn't thrown you can see the finally block message in the console.

xxxxxxxxxx
 
try {
  // insert your code here...
  console.log('Try block executed.');
} catch (error) {
  console.error(error);
} finally {
  console.log('Finally block message (executed on success and exception).');
}

Auto running

3. When exception is thrown but not catched, the finally block will still execute.

xxxxxxxxxx
 
try {
  const x = 3;
  if (x != 0) {
    throw 'Custom error message.'
  }
  console.log('Try block executed.');
} finally {
  console.log('Finally block message (executed on success and exception).');
}