Error, Error Handling and Error Handling Technique
Tags: webdev, errors, javascript, programming
Published: 2023-05-04 13:35:08

## **Error**
An error in software development  is an undesired or unexpected event that occurs during the execution of a program or application. In software development, an error is every developer's nightmare and one of the reasons why people give up on software development.

## **Error Types**
They are various types of errors associated with software development but the  most dominant type of errors are

- Operational Error

- Functional Error

## **Operational Error**
Operational Error in software development refers to runtime errors an application or a program encounters due to unhandled exception or another code issue. These errors can be challenging to diagonise as memory leaks, infinite loops, incorrect system configuration or a combination of these issues can be a cause of  operational errors.

## **Functional Error**
Functional error refers to bugs in the application code that prevents the program or application from performing as expected. these types of error usually require more effort to identify than operational error. The major causative of Functional error are incorrect logic, misused use cases, incorrect syntax, typos and misconfiguration.

## **Error Handling**

Error handling is the process of identifying, responding and recovering from errors that occurs during the execution of a program or application. The way a developer handles an application error says a lot about the developer. A good developer must be a good error handler.

## **Error Handling Techniques**
Error Handling techniques are those approach a developer uses in handling errors. the technique used in handling various errors depends greatly on the type of programming you are writing and the use case. The error technique for a security application might not be the same for an e-commerce website. they are various error handling techniques and they include

1. Try, Catch and Finally Block
2. The Call-back Function
3. Promises
4. Async and Await
5. Event Emitters.

## **Try, Catch and Finally Block**
.
the `Try`, `Catch` and `Finally` block is a programming construct used for error handling. This construct are divided into the three blocks.

**Try block:** the `try` block is used to enclose an code which might throw an error during execution. The purpose of this is to `try` the code and catch any error that might be in the code. When a `try` block is executed, the JavaScript runtime attempts to execute each statement inside the block in order. If an error occurs during the execution of any statement, control is immediately transferred to the nearest `catch` block that can handle the type of error that occurred. If no matching `catch` block is found, the error is thrown out of the `try` block and caught by any enclosing `catch` block, or the program crashes if no enclosing `catch` block is found.

**Catch Block:** The `catch` block is the next block of code after the the `try` block, the `catch` block is used to handle errors that might have be thrown by the `try` block. The `catch` block takes an error as as a parameter which contain the information about the error thrown by the `try` block and inside the `catch` block, developers can write code that handles the error in a controlled and predictable way, such as logging an error message, displaying a user-friendly error message, or attempting to recover from the error.

**Finally Block:** The `finally` block is an optional block after the `catch` block, this block of code are used to implement functions that will be executed regardless of whether an error is thrown or not. The main purpose of the `finally` block is to perform any necessary cleanup tasks, such as releasing resources like file handles or network connections, that were acquired in the `try` block. This ensures that resources are properly released even if an error occurs.

```
try {
  client = await MongoClient.connect(uri);
    // Insert the name into the collection
    const result = await collection.create();
    console.log('Name added to the database:', result.ops[0].name);
  } catch (err) {
    console.log('An error occurred while adding the name to the database:', err);
  } finally {
    // Close the database connection
    client.close();
  }
}); 
```
This code is an example of using a try-catch-finally block in Node.js to add a name to a MongoDB database.

The `try` block contains the code that attempts to insert a new document into the names collection using the `create` method. The `await` keyword is used to make the `create` method return a promise, which allows us to use the `async/await` syntax for handling asynchronous operations in a synchronous style.

If an error occurs during the execution of the `try` block, the error object is caught by the `catch` block, which logs an error message to the console.

The finally block is used to close the MongoDB connection using the close method of the `MongoClient` instance. This ensures that the database connection is always closed, regardless of whether the operation was successful or not.

By using a `finally` block to close the database connection, we ensure that our application is properly cleaning up after itself and releasing any resources that were allocated during the operation. This is important to prevent resource leaks and ensure that our application is efficient and reliable.

## **Async and Await**
in JavaScript, `async` and `await` keywords are used for handling asynchronous operation. the `async` keyword is used to define an asynchronous function which returns a promise. the `await` keyword can then be used in the function to wait for the completion of the asynchronous function before moving on to the next line of code. if an error occurs during the execution of the `await` statement, it will throw an error and control will be passed to the nearest `catch` block.

```
async function addNameToDatabase(collection, nameToAdd) {
  let client;

  try {
    // Connect to the database
    client = await MongoClient.connect(uri);

    // Insert the name into the collection
    const result = await collection.insertOne(nameToAdd);
    console.log('Name added to the database:', result.ops[0].name);
  } catch (err) {
    console.error('An error occurred while adding the name to the database:', err);
  } finally {
    // Close the database connection
    if (client) {
      client.close();
    }
  }
}

```
In this code, we've defined an async function called `addNameToDatabase` that takes two arguments: a collection object and a `nameToAdd` string. Inside the function, we've declared a variable called client to store our database connection.

The try block contains the code that inserts the `nameToAdd` string into the collection object. We're using the await keyword to wait for the `insertOne` method to complete before moving on to the next line of code. If the `insertOne` method throws an error, the catch block will catch it and log an error message to the console.

Finally, we have a `finally` block that closes the database connection. We're using an if statement to check whether the client object exists before calling the close method, in case an error occurred before the connection was established.

To use this function, you can simply call it with your collection and `nameToAdd` arguments:

```
const collection = db.collection('names');
const nameToAdd = 'John';

addNameToDatabase(collection, nameToAdd);

```
In summary, we've converted the original code into an async function that makes use of the await keyword to handle asynchronous code, and added error handling and a finally block to ensure that the database connection is closed properly.

##**Promises**
In error handling, promises are often used to handle errors that occur during asynchronous operations such as network requests or database queries. When a promise is resolved successfully, its `then()` method is called, and the result of the operation is passed to the next part of the program. If an error occurs during the operation, the promise is rejected, and the error is passed to the promise's `catch()` method, which can be used to handle the error and prevent the program from crashing.

```
function addNameToDatabase(collection, nameToAdd) {
  return MongoClient.connect(uri)
    .then((client) => {
      // Insert the name into the collection
      return collection.insertOne(nameToAdd)
        .then((result) => {
          console.log('Name added to the database:', result);
          // Close the database connection
          client.close();
        })
        .catch((err) => {
          console.log('An error occurred while adding the name to the database:', err);
          // Close the database connection
          client.close();
          throw err;
        });
    })
    .catch((err) => {
      console.log('An error occurred while connecting to the database:', err);
      throw err;
    });
}

```

Here's an example of how to use promises to handle the error in the `addNameToDatabase` function
In this version of the function, we're returning a promise from `addNameToDatabase`, which resolves when the name is successfully added to the database, and rejects if any errors occur. We're also using promises to handle the connection to the database and the closing of the database connection.

Inside the then block that's called when `MongoClient.connect(uri)` resolves, we're returning another promise that resolves when the name is successfully added to the database, and rejects if any errors occur. We're also using a catch block to handle any errors that occur during the insertion of the name, and to make sure that the database connection is closed before the error is re-thrown.

Finally, we're using another `catch` block outside the `then` block to handle any errors that occur during the connection to the database, and to make sure that the error is re-thrown so that it can be handled by any code that's calling addNameToDatabase.

##**Good Error Handling Practices**

1. Identifying potential source of errors.
*  Anticipating possible errors and taking steps to prevent them.
*  Providing clear and informative error messages that helps users and developers understand what went wrong and how to fix it.