JavaScript Error & Exception handling

An Error is an illegal operation that sometimes leads to the abnormal working of code or code failure, there are many types of error defined in computer science. An exception is an event we can also call an error, but an exception only occurs during the execution of the program, these are also known as runtime errors. And exception handling is a mechanism that handles runtime error so the flow of the program does not disrupt. JavaScript also provides the mechanism for exception handling which is similar to the if...else statements, where if an exception occurs in a specific block, we execute another block of code to handle that error.

JavaScript Error or Exception handling

JavaScript provides 4 keywords that represent a block of code using curly brackets to handle exceptions or errors.

try
catch
throw
finally

The try block represents the code block that possibly contains any error. The catch block represents that code that should execute if the try block has an error. In simple word, it handles the error if there is an error in try block. The throw block allows us to create a custom error. We can use throw to tell the user about our own error messsage. The finally block always gets executed after the try or catch block. It is an optional code block.

1. JavaScript try and catch

The try and catch block are always used together like if and else statements. The try statement or block contain the main code that may have some exception or error during execution. The catch block allow us to define a block of code that should be executed when there is an error in the try block. try and catch syntax

 try {
  //try code block
}
catch(err) {
  //catch code block
}

try and catch in action

Now let's see how the try and catch statements works. Now we will write an error code in try block and to handle that error we will write some console error messages in the catch block.

<script>
    try
    {
        console.logg("Hello World welcome to tecgheekbuzz"); //  error
    }

    catch(err)
    {
        console.log("There is some error in the try statement");
    }
</script>

Output(console)

There is some error in the try statement

In the above example inside the try block the console.logg() function created an error because there is no such console.logg() function in JavaScript. When the JS engine finds the error in the try block it automatically executed the catch block. And in output, we receive catch console print. The err statement in the catch(err) is the actual error message, if you want you can also print it on the console using console.log() function.

2. JS throw errors

Whenever the JavaScript engine or interpreter detects an error it stops executing the further statements. Although many of the errors are predefined inside the JS engine, but using the throw keyword we can create our own error.

Syntax

throw "error message"

Whenever the JS interpreter executes the throw statement it throws a custom error with the throw error message and stops the execution of the program there only. Example

<script>
    console.log("Hello World");
    throw "This is a Custom error";
    console.log("Hello World");
</script>

Output you can see the output on the console window error section.

3. JS finally statement

The finally is also a block of code, it is an optional block that used after the try and catch statement. And it gets executed anyhow after the complete code execution of try or catch block. Syntax

try {
  //try block
}
catch(err) {
  //catch block
}
finally {
  //finally block
}

Example

<script>
    try
    {
        console.logg("Hello World welcome to tecgheekbuzz"); //  error
    }
    catch(err)
    {
        console.log("There is some error in the try statement");
    }
    finally
    {
        console.log("This statement will execute anyhow")
    }
</script>

Output(console)

There is some error in the try statement
This statement will execute anyhow

Summary

Exceptions are the errors that occur during runtime.
try block represents the code that may contain exceptions.
The catch block represents the code that should handle exceptions of the try block.
Using the throw keyword we can throw custom errors.
The finally statement used after the catch statement executes regardless of the try and catch statements.

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