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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
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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
tryblock and executes the code in thecatchblock to handle this exception. - If the code executes without any errors, it skips the
catchblock and executes the code that comes after thecatchblock (sequential execution).
- If the code throws an error/exception, it stops the execution in the
-
catch: The code block that follows this statement handles the error/exception thrown by thetryblock and responds appropriately without crashing the program.
In JavaScript, we can add a throw or a finally clause with these statements.
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throw: This statement is used within thetryblock. We can use these statements to write the errors. For example, if we expect multiple types of errors in thetryblock, we can handle all these errors differently by usingthrowstatements. -
finally: The code in this block will run after thetryand/orcatchblocks 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 herethrow 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
erroron the console.
Outside try-catch block:
Line 9: We display the message
End of try-catch blockon 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 statementsnumber = 5if(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 hereconsole.log(err.message)}else if(err.message == 'Try again') {// we can write code to handle this error hereconsole.log(err.message)}}console.log("End of try-catch block")
Note: We can obtain our error message by accessing the
messageattribute of theErrorobject.
Explanation
try block:
Line 3: We initialize the value of
numberto5.Lines 5-8: We
throwthe errors based on the value ofnumber. In our case the value ofnumberis5so wethrowthe error with the messageTry again.
catch block:
Lines 12-20: We identify our error using
err.messageand print theerror'smessageon the console. In our case theerr.messageisTry again.
Outside try-catch block:
Lines 23: We display the message
End of try-catch blockon 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 workson the console.Line 3: We
throwour own error that saysThis statement throws an new error. This error will be caught in thecatchblock.
catch block:
Line 6: We display the message
Error has been handledon the console.
finally block:
Line 9: We display the message
Everything has been handledon 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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