How to use try-catch statements in JavaScript

Exception handling is the a of responding to unexpected events during the execution of a computer program. This ensures that the program runs smoothly.

Let’s discuss the role of the try-catch statement in exception handling.

The try-catch block

  • try: The program will run the code block that follows this statement and does one of the following:

    • If the code throws an error/exception, it stops the execution in the try block and executes the code in the catch block to handle this exception.
    • If the code executes without any errors, it skips the catch block and executes the code that comes after the catch block (sequential execution).

  • catch: The code block that follows this statement handles the error/exception thrown by the try block and responds appropriately without crashing the program.

In JavaScript, we can add a throw or a finally clause with these statements.

  • throw: This statement is used within the try block. We can use these statements to write the errors. For example, if we expect multiple types of errors in the try block, we can handle all these errors differently by using throw statements.

  • finally: The code in this block will run after the try and/or catch blocks have completed their execution.

Syntax

Now that we know what each statement does and its use, let’s look at the syntax below:

try{
  // Code to test for error comes here
  throw new Error ("some error") // We can make our own errors and handle them in the catch block
}
catch(error){
  //Code to handle error comes here
}
finally{
  //Code to be executed after the code in try and/or catch blocks is executed
}

Examples

Let’s look at a few examples of exception handling in JavaScript using the try, catch, throw, and finally statements.

Using a simple try-catch statement

In the example below, we will deliberately write a statement giving an error in the try block to show how it works. Then, we will log the error to the console in the catch block:

try{
  addAlert()  
}
catch(error){ 
  console.log(error)  
}
console.log("\nEnd of try-catch block")

Explanation

try block:

  • Line 2: We call the addAlert() method. This will throw an exception as we haven't declared this function.

catch block:

  • Line 6: We display the error on the console.

Outside try-catch block:

  • Line 9: We display the message End of try-catch block on the console. This statement will be executed after the try-catch block.

Using the try, throw, and catch statements

The example below shows how we can create our error using a throw statement. It also shows how we can create different errors and how to handle them differently in the catch block.

try{
  // you can change this number to see how the try-catch block works with throw statements
  number = 5
  if(number == 2)
    throw new Error ('This is the wrong input')
  else if(number == 5)
    throw new Error ('Try again')
}
catch(err){
  if(err.message == 'This is the wrong input')
    {
      // we can write code to handle this error here
      console.log(err.message)
    }
  else if(err.message == 'Try again') {
    // we can write code to handle this error here
    console.log(err.message)
  }
}
console.log("End of try-catch block")

Note: We can obtain our error message by accessing the message attribute of the Error object.

Explanation

try block:

  • Line 3: We initialize the value of number to 5.

  • Lines 5-8: We throw the errors based on the value of number. In our case the value of number is 5 so we throw the error with the message Try again.

catch block:

  • Lines 12-20: We identify our error using err.message and print the error's message on the console. In our case the err.message is Try again.

Outside try-catch block:

  • Lines 23: We display the message End of try-catch block on the console. This statement will be executed after the execution of the try-catch block.

Using try, catch, throw, and finally together

This last example shows the use of the finally statement with the other three statements. We throw an exception using the throw statement in the try block. ​​Then, the program executes the catch block and the finally block in the end:

try{
  console.log("This statement works")
  throw new Error('This statement throws an error')
}
catch(error){
  console.log("Error has been handled")
}
finally{
  console.log("Everything has been handled")
}

Explanation

try block:

  • Line 2: We display the message This statement works on the console.

  • Line 3: We throw our own error that says This statement throws an new error. This error will be caught in the catch block.

catch block:

  • Line 6: We display the message Error has been handled on the console.

finally block:

  • Line 9: We display the message Everything has been handled on the console. This will be executed in the end after the execution of try-catch blocks.

Conclusion

It is important for us to know how to use try-catch, finally and throw statements in JavaScript to handle our program's errors more efficiently. In this way, we can handle the errors on our own without facing any program crashes.

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