Handling errors in JavaScript

There are 3 types of errors in the program: (a) syntax error (Syntax Error), (b) error while running the program (Runtime Error), and (c) error of logic of the program structure (Logical Error) .

Syntax Error

Syntax Error, also called parsing error, occurs at compile time in traditional program languages ​​and at interpreting time in JavaScript.

For example, the following line causes a syntax error because it lacks closed parentheses.

  type = "text/javascript" >  

When a syntax error occurs in JavaScript, only the code that contains the same thread is affected and the rest of the code in the other thread is still executing when assuming it does not depend on the code containing the error.

Runtime Error

Runtime Error, also called Exceptions, occurs during execution (after compiling / interpreting).

For example, the following line creates a Runtime Error because here the syntax is correct, but while running, it tries to call a method that does not exist.

  type = "text/javascript" >  

Runtime Error also affects the thread in which they occur, allowing another thread in JavaScript to continue executing normally.

Logical Error

Logical errors are a type of error that is hard to find traces. These errors are not the result of syntax or error while running. Instead, they happen when you create a logical error that controls your script and you don't get the expected results.

You cannot catch these errors, because it depends on the requirements and logic types you put into the program.

The try . catch . finally command

The latest version of JavaScript adds the ability to handle program run errors. JavaScript implements try . catch . finally as well as throw operators to handle Runtime Error.

You can catch the Runtime Error but you cannot catch the syntax errors (Syntax Error).

The try . catch . finally block syntax is as follows:

  type = "text/javascript" >  

The try block must be followed by either a catch block or a finally block (or either). When a Runtime Error occurs in the try block, this error is placed in e and the catch block is executed. Finally block optionally executes unconditionally after try / catch conditions .

For example

Here is an example where we try to call a non-existent function that creates Runtime Error. We see how it executes without using try . catch :

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

Now we try to catch this error by using try . catch and displaying a user-friendly message (user-fiendly). You can also dismiss this message, if you want to hide this error, do not show it to the user.

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

You can use finally blocks which will always execute unconditionally after try / catch . Here is an example:

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

Throw command

You can use the throw command to raise available Runtime Error errors or your custom errors. Then these errors can be captured and you can make a reasonable action.

For example

The following example illustrates how to use a throw command.

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

You can raise an error in a function by using a string, integer, logic, or an object and then you can catch this error or in the same function as we did above, or follow another function. by using try . catch block.

Onerror method ()

The onerror method is the first feature to handle errors in JavaScript. The error event is triggered on the window object whenever an exception occurs on the page.

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

The onerror error handling method provides 3-part information to determine the exact error.

Error message - The notification that the browser will display with a given error.

URL - File where the error occurred.

Line number - Which line in the given URL causes an error.

Here is an example of how to extract this information:

For example

  type = "text/javascript" >  

 Click the following to see the result:  type = "button" value = "Click Me" onclick = " myFunc (); " /> 

Result

Run the above command to see the result

You can display the extracted information in any way you think it is good.

You can use an onerror method, as shown below, to display an error message in case of any problems while loading an image.

  src = "myimage.gif" onerror = " alert ( 'An error occurred loading the image.' ) " /> 

You can use onerror with multiple HTML tags to display appropriate messages in case of an error.

According to Tutorialspoint

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