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JavaScript - try / catch statement

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

Introduction

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

Syntax / exampleDescription
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.
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.
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

Try... catch block

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:

// ONLINE-RUNNER:browser;

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

Throw statement

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

// ONLINE-RUNNER:browser;

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

Note:

You can throw different types of exceptions.

Finally statement

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.

// ONLINE-RUNNER:browser;

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).');
}

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

// ONLINE-RUNNER:browser;

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).');
}

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

// ONLINE-RUNNER:browser;

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).');
}

References

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