Handling errors or exceptions is an integral part of software application development. Errors or exceptions can cause the program to work in an unexpected manner, resulting in security issues or inconvenience to the user. For this reason, it is very critical to handle errors appropriately.  Let’s consider a simple software program like a calculator performing division of two numbers. The software provides an input screen for users to enter the data on which the operation is required to be performed. Now, what if the user types in an invalid input, like say text instead of numbers or tries to perform division by zero. What happens if an error occurs?  Any software is designed to perform orderly transitions between different states. Now because of errors, we might end up being in a state which is not considered or envisaged during software development. This situation is known as unpredictable state of the software and can lead to serious vulnerabilities such as loss of data, buffer overflows etc.  Error handling, also called as exception handling, will help us prevent this unpredictable state by providing explicit instructions on handling unpredictable states. For example, if the user enters string data instead of numeric data for performing division operation, we can validate the data by transitioning to validation state for checking conditions that might create an error. If there is no error, software would continue to execute, otherwise it would enter a state where it would display the error message. Like most programming languages, JavaScript also has exception handling statements i.e. throw statement and try...catch statement. A note on exception Types: JavaScript provides predefined exceptions for specific purposes and it is suggested to use one of them for effectiveness. That said, any object can be thrown in JavaScript and the most common practice is to throw numbers or strings as errors.  throw statement is used to throw an exception with a specified value to be thrown. A few examples are as follows: throw 'new error';  // String type  throw 54;           // Number type  throw false;        // Boolean type  throw { errorcode: 500, message: 'internal server error' } // Object type try…catch statement consists of a try block which contains multiple statements and a catch block containing the statements to be executed if an exception happens in try block during execution of statements in that block. In a positive flow, all the statements in try block succeed and control will skip the catch block. Otherwise, if there is an exception thrown in the try block, the following lines in try block are skipped and the control shifts to catch. In either case, finally block executes after try and catch blocks are executed. function getIPLTeamCaptain(team){    let teams = new Map();    teams.set('CSK', 'Dhoni');    teams.set('RCB', 'Kohli');    teams.set('SRH', 'Warner');    teams.set('MI', 'Rohit');    teams.set('KXI', 'Rahul');    teams.set('DC', 'Pant');    teams.set('KKR', 'Morgan');    teams.set('RR', 'Sanju');    if (teams.has(team) === true) {      return teams.get(team);    } else {      throw 'InvalidTeam';    }  }  try {    const captain = getIPLTeamCaptain('Pune');    console.log(captain);  } catch (error) {    console.error(error);  } In catch block, we have an identifier that holds the value specified by the throw statement. This can be used to get the information about the exception details. The scope of this identifier lasts within catch block and post finishing the execution, the identifier no longer exists. Note on best practices: When logging errors to the console inside a catch block, it is advisable to use console.error() rather than console.log() for debugging. It formats the message as an error, and adds it to the list of error messages generated by the page. Few pitfalls or best practices to be known while considering finally block If there is an I/O operation in try block which results in an exception, then it is better to handle the clean-up code in finally block. For example, a DB connection is made or a file is opened and during processing of the file an exception has occurred. Though the catch will handle the exception, clean up code has to be handled in finally block i.e. closing of file. The finally block will execute regardless of whether an exception has been raised or not.  If a finally block returns a value, then regardless of whether an exception is raised or a value is returned in the try and catch block, the finally block return value will overwrite those values. function raiseException() {    try {      throw 500;    } catch(e) {      console.error(e);      throw e; // this throw statement is suspended until finally block has completed    } finally {      return false; // overwrites the previous "throw"    }    // always returns false  }  try {    raiseException();  } catch(e) {    // this is never reached!    // while raiseException() executes, the finally block returns false, which overwrites the throw inside the above catch    console.error(e);  } Nesting of try..catch statements is possible; but if inner try block does not have corresponding catch block then it should have a finally block. The enclosing try…catch statement’s catch block is checked for a match. Error objects are a specific type of core objects which are thrown during runtime. To log more refined messages about error, we can use ‘name’ and ‘message’ properties. Refer to this link to know more about Error objectFor example, throw new Error(‘Error Message’); when caught in catch block, the identifier will have the property name as ‘Error’ and property message as ‘Error Message’. When a runtime error occurs, error objects are created and thrown. Any user-defined exceptions can be built extending the Error. Below are some of the predefined error constructors extended from generic Error constructor.  AggregateError  EvalError  RangeError  ReferenceError  SyntaxError  TypeError  URIError  Let’s understand each one with an example. 1. AggregateError as the name suggests is used to wrap multiple errors into a single error. Imagine we are processing multiple async calls via promises and used Promise.any(). This would raise AggregateError or we can create our own new AggregateError as shown below. Promise.any([    Promise.reject(new Error("some error")),  ]).catch(e => {    console.log(e instanceof AggregateError); // true    console.log(e.message);                   // "All Promises rejected"    console.log(e.name);                      // "AggregateError"    console.log(e.errors);                    // [ Error: "some error" ]  }); OR try {    throw new AggregateError(['Error', 500, new Error('message')],'AggregateErrorMessage');  } catch (e) {    console.log(e instanceof AggregateError); // true    console.log(e.message);                   // "AggregateErrorMessage"    console.log(e.name);                      // "AggregateError"   console.log(e.errors.length)              // 3  } 2. EvalError occurs when using global eval() function. This exception is no more thrown by JavaScript. 3. RangeError is to be thrown when a value is not in the range of allowed values. Like passing bad values to numeric methods like toFixed, toPrecision etc. try {    var num = 10.123.toFixed(-1);  } catch (e) {    console.error(e instanceof RangeError); // true    console.error(e.message);               // argument must be between 0-100    console.error(e.name);                  // RangeError  } 4. ReferenceError is raised when a non-existent variable is referenced. try {    let a = undefinedVar  } catch (e) {    console.log(e instanceof ReferenceError)  // true    console.log(e.message)                    // "undefinedVar is not defined"    console.log(e.name)                       // "ReferenceError"  } 5. SyntaxError is thrown by the JavaScript engine if it encounters tokens which do not conform to the syntax of the language while parsing the code. try {    eval('hoo bar');  } catch (e) {    console.error(e instanceof SyntaxError); // true    console.error(e.message);                // Unexpected identifier    console.error(e.name);                   // SyntaxError  } 6. TypeError is raised if an operation is performed on a value which is not expected on its type. Say if we are attempting to modify a value that cannot be changed. try {    const a = "constant";    a = 'change';  } catch (e) {    console.log(e instanceof TypeError)  // true    console.log(e.message)               // "Assignment to constant variable."    console.log(e.name)                  // "TypeError"  } 7. URIError is raised when global URL handling function is used in an inappropriate way. try {    decodeURIComponent('%')  } catch (e) {    console.log(e instanceof URIError)  // true    console.log(e.message)              // URI malformed    console.log(e.name)                 // "URIError  }  Handling Specific errorIf we want to handle different error types differently then we can check for error instanceof and handle them accordingly. try {    customMethod();  } catch (e) {    if (e instanceof EvalError) {      console.error(e.name + ': ' + e.message)    } else if (e instanceof RangeError) {      console.error(e.name + ': ' + e.message)    } else if (e instanceof URIError) {      console.error(e.name + ': ' + e.message)    } else if (e instanceof TypeError) {      console.error(e.name + ': ' + e.message)    } else if (e instanceof AggregateError) {      console.error(e.name + ': ' + e.message)    } else if(typeof e === 'string' || e instanceof String) {      console.error(Error of type string with message ${e}); } else if(typeof e === 'number' || e instanceof Number) { console.error(Error of type number with message${e});    } else if(typeof e === 'boolean' || e instanceof Boolean) {      console.error(Error of type boolean with message ${e}); } else { // if we aren't sure of other types of errors, // probably it is best to rethrow throw e; } } Note: In real time applications, we would handle specific error types based on the functionality in the try block and wouldn’t have so many different error types for a single try...catch block. Also having multiple errors and having multiple if … else statements is not recommended as per the single responsibility principle. Please refer to the above code only for reference to identify the specific error type. Custom Error TypeSometimes we would like customize the error or want to have our own error types. To create custom Error types, we have to extend the existing Error object and throw the custom error accordingly. We can capture our custom error type using instanceof. This is a cleaner and more consistent way of error handling. class MyError extends Error { constructor(customProperty, ...params) { // Pass arguments to parent constructor super(...params); // Maintains proper stack trace for where our error was thrown(available on V8) if (Error.captureStackTrace) { Error.captureStackTrace(this, MyError); } this.name = 'MyError'; // Custom debugging information this.customProperty = customProperty; this.date = new Date(); } } try { throw new MyError('custom', 'message') } catch(e) { console.error(e.name) // MyError console.error(e.customProperty) // custom console.error(e.message) // message console.error(e.stack) // MyError: message with stacktrace } DOMException TypeThe DOMException occurs as a result of calling a method or accessing a property of a web API which represents an abnormal event that occurred. Refer to this link to know more about DOMException Promise Error PropagationA Promise is an object representing the eventual completion or failure of an asynchronous operation. Promises allow to chain multiple asynchronous operations back-to-back in a sequence order. When we chain multiple asynchronous operations, then we can have catch at the end of the chain to capture any exception or rejection that happened at any promise. Catch takes a callback function and the callback is set to execute when the Promise is rejected. For example: Promise.resolve(doSomething) .then(action1 => Action1(action1)) .then(action2 => Action2(action2)) .catch(failureCallback); // Attaches a callback for only the rejection of the Promise. // @returns — A Promise for the completion of the callback. As per the above example, we are performing multiple async actions but we have a single catch block to capture any rejection. Let’s understand this with an assumption that the calls are synchronous in nature. try { const action1 = doSomething(); const action2 = Action1(action1); const finalResult = Action2(action2); console.log(Got the final result:${finalResult});  } catch(error) {    failureCallback(error);  } If there’s an exception, then the browser will jump over to catch block and execute the failureCallback. Based on ECMAScript 2017 async/await syntactic sugar-coated way, we can change the synchronous code to asynchronous. async function SampleAsync() {    try {      const action1 = await doSomething();      const action2 = await Action1(action1);      const finalResult = await Action2(action2);      console.log(Got the final result: ${finalResult}); } catch(error) { failureCallback(error); } } Promises help to resolve the callback hell by catching all errors i.e. thrown exceptions or programming errors. This is important for functional composition of async operations. What would happen if there isn’t a catch for a promise? Promises are associated with two events i.e. rejectionhandled and unhandledrejection. Whenever a promise is rejected, one of these two events is sent to the global scope which might be a window or a worker. rejectionhandled - sent when a promise is rejected, and post rejection the executor’s reject is handled. unhandledrejection – promise is rejected but there is no rejection handler available. Note: Both events are of type PromiseRejectionEvent which has details of the actual promise which was rejected and a reason property depicting the reason for rejection. Since both the events are in global scope, all errors will go to the same event handlers regardless of the source. This makes it easy to offer a fallback error handling mechanism for promises. While working with Node.js on the server side, we might include some common modules and there is a high possibility that we may have some unhandled rejected promises which get logged to the console by the Node.js runtime. If we want to capture those and process or log them outside the console, then we can add these handlers on the process as shown below. process.on("unhandledRejection", (reason, promise) => { // You might start here by adding code to examine the "promise" and "reason" values. }); The impact of adding this handler on the process is that it will prevent the errors from being logged to the console by the node.js runtime. There is no need for calling preventDefault() method available on browser runtimes. Note: Adding the listener on the process.on and not coding to capture the promise and reasons would result in loss of the error. So it is suggested to add code to examine the rejected promise to identify the cause of rejection. Window UnhandledRejection EventThe global scope being window (may also be worker), it is required to call the event.preventDefault() in the unhandledrejection to cancel the event. This prevents it from bubbling up to be handled by the runtime’s logging code. It works because unhandledrejection is cancellable. window.addEventListener('unhandledrejection', function (event) { // ...your code here to handle the unhandled rejection... // Prevent the default handling (such as outputting the // error to the console) event.preventDefault(); }); To summarise a few points related to error handling: Try to handle errors specific to the type of error. Use Error object instead of numeric, string or Boolean while throwing an error. As Error object will capture the stack trace and error type, specific handling is possible. Release resources if any in finally block. Being more specific with throw error helps in handling it cleanly instead of generic error. Avoid unwanted catch block if they are meant only for logging, and re-throw the error again. Otherwise, this will result in clogging of logs. Last but not least, suppression/swallow/shadowing of exception as shown below should be avoided. This might make the application continue to work but the underlining issue will not be fixed. try { throw new Error('my error'); } catch(e) {} // no one knows about this try { throw new Error('my error'); } catch(e) { console.log(err); // error is logged but application continues } try { throw new Error('my error'); } finally { return null; // error is swallowed } try { throw new Error('actual error'); } finally { throw new Error('new error'); // actual error is shadowed by new one } Conclusion Error handling is one of the most important facets in software programming that has to be acknowledged and experienced by the developer. Software should be as predictable as possible in terms of state transitions. # How to Handle Errors in JavaScript 9K Handling errors or exceptions is an integral part of software application development. Errors or exceptions can cause the program to work in an unexpected manner, resulting in security issues or inconvenience to the user. For this reason, it is very critical to handle errors appropriately. Let’s consider a simple software program like calculator performing division of two numbers. The software provides an input screen for users to enter the data on which the operation is required to be performed. Now, what if the user types in an invalid input, like say text instead of numbers or tries to perform division by zero. What happens if an error occurs? Any software is designed to perform orderly transitions between different states. Now because of errors, we might end up being in a state which is not considered or envisaged during software development. This situation is known as unpredictable state of the software and can lead to serious vulnerabilities such as loss of data, buffer overflows etc. Error handling, also called as exception handling, will help us prevent this unpredictable state by providing explicit instructions on handling unpredictable states. For example, if the user enters string data instead of numeric data for performing division operation, we can validate the data by transitioning to validation state for checking conditions that might create an error. If there is no error, software would continue to execute, otherwise it would enter a state where it would display the error message. Like most programming languages, JavaScript also has exception handling statements i.e. throw statement and try...catch statement. A note on exception Types: JavaScript provides predefined exceptions for specific purposes and it is suggested to use one of them for effectiveness. That said, any object can be thrown in JavaScript and the most common practice is to throw numbers or strings as errors. throw statement is used to throw an exception with a specified value to be thrown. A few examples are as follows: throw 'new error'; // String type throw 54; // Number type throw false; // Boolean type throw { errorcode: 500, message: 'internal server error' } // Object type  try…catch statement consists of a try block which contains multiple statements and a catch block containing the statements to be executed if an exception happens in try block during execution of statements in that block. In positive flow, all the statements in try block succeed and control will skip the catch block. Otherwise, if there is an exception thrown in the try block, the following lines in try block are skipped and the control shifts to catch. In either case, finally block executes after try and catch blocks are executed. function getIPLTeamCaptain(team){ let teams = new Map(); teams.set('CSK', 'Dhoni'); teams.set('RCB', 'Kohli'); teams.set('SRH', 'Warner'); teams.set('MI', 'Rohit'); teams.set('KXI', 'Rahul'); teams.set('DC', 'Pant'); teams.set('KKR', 'Morgan'); teams.set('RR', 'Sanju'); if (teams.has(team) === true) { return teams.get(team); } else { throw 'InvalidTeam'; } } try { const captain = getIPLTeamCaptain('Pune'); console.log(captain); } catch (error) { console.error(error); }  In catch block, we have an identifier that holds the value specified by the throw statement. This can be used to get the information about the exception details. The scope of this identifier lasts within catch block and post finishing the execution, the identifier no longer exists. Note on best practices: When logging errors to the console inside a catch block, it is advisable to use console.error() rather than console.log() for debugging. It formats the message as an error, and adds it to the list of error messages generated by the page. Few pitfalls or best practices to be known while considering finally block • If there is an I/O operation in try block which results in an exception, then it is better to handle the clean-up code in finally block. For example, a DB connection is made or a file is opened and during processing of the file an exception has occurred. Though the catch will handle the exception, clean up code has to be handled in finally block i.e. closing of file. • The finally block will execute regardless of whether an exception has been raised or not. If a finally block returns a value, then regardless of whether an exception is raised or a value is returned in the try and catch block, the finally block return value will overwrite those values. function raiseException() { try { throw 500; } catch(e) { console.error(e); throw e; // this throw statement is suspended until finally block has completed } finally { return false; // overwrites the previous "throw" } // always returns false } try { raiseException(); } catch(e) { // this is never reached! // while raiseException() executes, the finally block returns false, which overwrites the throw inside the above catch console.error(e); }  Nesting of try..catch statements is possible; but if inner try block does not have corresponding catch block then it should have finally block. The enclosing try…catch statement’s catch block is checked for a match. Error objects are specific type of core objects which are thrown during runtime. To log more refined messages about error, we can use ‘name’ and ‘message’ properties. Refer to this link to know more about Error object For example, throw new Error(‘Error Message’); when caught in catch block, the identifier will have the property name as ‘Error’ and property message as ‘Error Message’. When a runtime error occurs, error objects are created and thrown. Any user-defined exceptions can be built extending the Error. Below are some of the predefined error constructors extended from generic Error constructor. • AggregateError • EvalError • RangeError • ReferenceError • SyntaxError • TypeError • URIError Let’s understand each one with an example. 1. AggregateError as the name suggests is used to wrap multiple errors into a single error. Imagine we are processing multiple async calls via promises and used Promise.any(). This would raise AggregateError or we can create our own new AggregateError as shown below. Promise.any([ Promise.reject(new Error("some error")), ]).catch(e => { console.log(e instanceof AggregateError); // true console.log(e.message); // "All Promises rejected" console.log(e.name); // "AggregateError" console.log(e.errors); // [ Error: "some error" ] });  OR try { throw new AggregateError(['Error', 500, new Error('message')],'AggregateErrorMessage'); } catch (e) { console.log(e instanceof AggregateError); // true console.log(e.message); // "AggregateErrorMessage" console.log(e.name); // "AggregateError" console.log(e.errors.length) // 3 }  2. EvalError occurs when using global eval() function. This exception is no more thrown by JavaScript. 3. RangeError is to be thrown when a value is not in the range of allowed values. Like passing bad values to numeric methods like toFixedtoPrecision etc. try { var num = 10.123.toFixed(-1); } catch (e) { console.error(e instanceof RangeError); // true console.error(e.message); // argument must be between 0-100 console.error(e.name); // RangeError }  4. ReferenceError is raised when a non-existent variable is referenced. try { let a = undefinedVar } catch (e) { console.log(e instanceof ReferenceError) // true console.log(e.message) // "undefinedVar is not defined" console.log(e.name) // "ReferenceError" }  5. SyntaxError is thrown by the JavaScript engine if it encounters tokens which do not conform to the syntax of the language while parsing the code. try { eval('hoo bar'); } catch (e) { console.error(e instanceof SyntaxError); // true console.error(e.message); // Unexpected identifier console.error(e.name); // SyntaxError }  6. TypeError is raised if an operation is performed on a value which is not expected on its type. Say if we are attempting to modifa value that cannot be changed. try { const a = "constant"; a = 'change'; } catch (e) { console.log(e instanceof TypeError) // true console.log(e.message) // "Assignment to constant variable." console.log(e.name) // "TypeError" }  7. URIError is raised when global URL handling function is used in an inappropriate way. try { decodeURIComponent('%') } catch (e) { console.log(e instanceof URIError) // true console.log(e.message) // URI malformed console.log(e.name) // "URIError }  ## Handling Specific error If we want to handle different error types differently then we can check for error instanceof and handle them accordingly. try { customMethod(); } catch (e) { if (e instanceof EvalError) { console.error(e.name + ': ' + e.message) } else if (e instanceof RangeError) { console.error(e.name + ': ' + e.message) } else if (e instanceof URIError) { console.error(e.name + ': ' + e.message) } else if (e instanceof TypeError) { console.error(e.name + ': ' + e.message) } else if (e instanceof AggregateError) { console.error(e.name + ': ' + e.message) } else if(typeof e === 'string' || e instanceof String) { console.error(Error of type string with message${e});
} else if(typeof e === 'number' || e instanceof Number) {
console.error(Error of type number with message ${e}); } else if(typeof e === 'boolean' || e instanceof Boolean) { console.error(Error of type boolean with message${e});
} else {
// if we aren't sure of other types of errors,
// probably it is best to rethrow
throw e;
}
} 

Note: In real time applications, we would handle specific error types based on the functionality in the try block and wouldn’t have so many different error types for a single try...catch block. Also having multiple errors and having multiple if … else statements is not recommended as per the single responsibility principle.

Please refer to the above code only for reference to identify the specific error type.

#### Custom Error Type

Sometimes we would like customize the error or want to have our own error types. To create custom Error types, we have to extend the existing Error object and throw the custom error accordingly. We can capture our custom error type using instanceof. This is cleaner and more consistent way of error handling.

class MyError extends Error {
constructor(customProperty, ...params) {
// Pass arguments to parent constructor
super(...params);
// Maintains proper stack trace for where our error was thrown(available on V8)
if (Error.captureStackTrace) {
Error.captureStackTrace(this, MyError);
}
this.name = 'MyError';
// Custom debugging information
this.customProperty = customProperty;
this.date = new Date();
}
}
try {
throw new MyError('custom', 'message')
} catch(e) {
console.error(e.name)           // MyError
console.error(e.customProperty) // custom
console.error(e.message)        // message
console.error(e.stack)          // MyError: message with stacktrace
} 

### DOMException Type

The DOMException occurs as a result of calling a method or accessing a property of a web API which represents an abnormal event that occurred.

### Promise Error Propagation

A Promise is an object representing the eventual completion or failure of an asynchronous operation. Promises allow to chain multiple asynchronous operations back-to-back in a sequence order.

When we chain multiple asynchronous operations, then we can have catch at the end of the chain to capture any exception or rejection that happened at any promise. Catch takes a callback function and the callback is set to execute when the Promise is rejected.

For example:

Promise.resolve(doSomething)
.then(action1 => Action1(action1))
.then(action2 => Action2(action2))
.catch(failureCallback);
// Attaches a callback for only the rejection of the Promise.
// @returns — A Promise for the completion of the callback. 

As per the above example, we are performing multiple async actions but we have a single catch block to capture any rejection. Let’s understand this with an assumption that the calls are synchronous in nature.

try {
const action1 = doSomething();
const action2 = Action1(action1);
const finalResult = Action2(action2);
console.log(Got the final result: ${finalResult}); } catch(error) { failureCallback(error); }  If there’s an exception, then the browser will jump over to catch block and execute the failureCallback. Based on ECMAScript 2017 async/await syntactic sugar-coated way, we can change the synchronous code to asynchronous. async function SampleAsync() { try { const action1 = await doSomething(); const action2 = await Action1(action1); const finalResult = await Action2(action2); console.log(Got the final result:${finalResult});
} catch(error) {
failureCallback(error);
}
}  

Promises help to resolve the callback hell by catching all errors i.e. thrown exceptions or programming errors. This is important for functional composition of async operations.

## What would happen if there isn’t a catch for a promise?

Promises are associated with two events i.e. rejectionhandled and unhandledrejection. Whenever a promise is rejected, one of these two events is sent to the global scope which might be a window or a worker.

• rejectionhandled - sent when a promise is rejected, and post rejection the executor’s reject is handled.
• unhandledrejection – promise is rejected but there is no rejection handler available.

Note: Both events are of type PromiseRejectionEvent which has details of the actual promise which was rejected and a reason property depicting the reason for rejection.

Since both the events are in global scope, all errors will go to the same event handlers regardless of the source. This makes it easy to offer a fallback error handling mechanism for promises.

While working with Node.js on the server side, we might include some common modules and there is a high possibility that we may have some unhandled rejected promises which get logged to the console by the Node.js runtime. If we want to capture those and process or log them outside the console, then we can add these handlers on the process as shown below.

process.on("unhandledRejection", (reason, promise) => {
// You might start here by adding code to examine the "promise" and "reason" values.
}); 

The impact of adding this handler on the process is that it will prevent the errors from being logged to the console by the node.js runtime. There is no need for calling preventDefault() method available on browser runtimes.

Note: Adding the listener on the process.on and not coding to capture the promise and reasons would result in loss of the error. So it is suggested to add code to examine the rejected promise to identify the cause of rejection.

## Window UnhandledRejection Event

The global scope being window (may also be worker), it is required to call the event.preventDefault() in the unhandledrejection to cancel the event. This prevents it from bubbling up to be handled by the runtime’s logging code. It works because unhandledrejection is cancellable.

window.addEventListener('unhandledrejection', function (event) {
// ...your code here to handle the unhandled rejection...
// Prevent the default handling (such as outputting the
// error to the console)
event.preventDefault();
}); 

To summarise a few points related to error handling:

• Try to handle errors specific to the type of error.
• Use Error object instead of numeric, string or Boolean while throwing an error. As Error object will capture the stack trace and error type, specific handling is possible.
• Release resources if any in finally block.
• Being more specific with throw error helps in handling it cleanly instead of generic error.
• Avoid unwanted catch block if they are meant only for logging, and re-throw the error again. Otherwise, this will result in clogging of logs.
• Last but not least, suppression/swallow/shadowing of exception as shown below should be avoided. This might make the application continue to work but the underlining issue will not be fixed.
try {
throw new Error('my error');
}
try {
throw new Error('my error');
}
catch(e) {
console.log(err); // error is logged but application continues
}
try {
throw new Error('my error');
}
finally {
return null; // error is swallowed
}
try {
throw new Error('actual error');
}
finally {
throw new Error('new error'); // actual error is shadowed by new one
} 

Conclusion

Error handling is one of the most important facets in software programming that has to be acknowledged and experienced by the developer. Software should be as predictable as possible in terms of state transitions.

### Sumanth Reddy

Author

Full stack, UI Architect having 14+ Years of experience in web, desktop and mobile application development with strong Javascript/.Net programming skills .

Strong experience in microsoft tech stack and Certified in OCI associate .

Go-to-guy on integration of applications, building a mobile app by zeroing on tech stack. Majorily experienced on engineering based IIOT products and Marekting Automation products deployed on premise and on-cloud.

## Back Up, Restore, and Migrate a MongoDB Database

Popular among both enterprises and startups, MongoDB is a database that is perfectly suited for web-apps that need to scale once the user base increases. MongoDB is different from traditional relational databases because it uses json like objects to store data, instead of tables in relational databases. In this post, we will learn to backup and restore a MongoDB database. In all software products there is an import and export feature, which in database terms, deals with human-readable format. On the other hand, the backup and restore operations use MongoDB specific data, which preserve the MongoDB attributes.  So, when migrating the database, we should prefer backup and restore over import and export. But we should also keep in mind that our source and target systems are compatible, which means that both should be Windows or both should be a Linux based system like Ubuntu/Mac. Prerequisites We are using Windows 10 in this tutorial. Please make sure you have downloaded the MongoDB Community Server and installed in it. It is a very easy setup and you will find lot of good articles on the internet detailing this out. Please ensure that you have added it in the Environment variable in your PC. Backup Considerations In a production environment, backups act as a snapshot of the database at a certain point. Large and complex databases do fail or can be hacked. If that happens, we can use the last backup file to restore the database to the point, before it failed. These are some of the factors which should be taken into consideration when doing a recovery.  1. Recovery Point Objective We should know the objective of the recovery point, which means how much data we are willing to lose during a backup and restoration. A continuous backup is preferred for critical data like bank information and backups should be taken several times during the day. On the other hand, if the data doesn’t change frequently, then the backup can be taken every 6 months.  2. Recovery Time ObjectiveThis tells how quickly the restoration can be done. During restoration the application will be down for some time; and this downtime should be minimized, or else customer relationships will be lost.  3. Database and Snapshot IsolationThis refers to the distance between the primary database server and the backup server. If they are close enough i.e., in the same building, then the recovery time reduces. However, in the event of a physical event such as a fire, there is a likelihood of it been destroyed with the primary database.   4. Restoration Process We should always test our backups in test servers to see if they will work, in case a restoration is required.  5. Available Storage Backup of database generally takes a lot of space and in most cases, it will never be required. So, we should try to minimize the space taken on the disk, by archiving the database into a zip file.  6. Complexity of DeploymentThe backup strategy should be easy to set and should be automated, so that we don’t have to remember to take the backup after regular intervals. Understanding the Basics The first thing that we should know is that MongoDB uses json and bson(binary json) formats for storing data. So, people coming from a JavaScript background can relate to objects for json, which have a key-value pair. Also, json is the preferred format in which we receive or send data to an API endpoint. You can check the json data of a MongoDB database in any tool or online editors. Even the famous Windows application Notepad++ has a json viewer. An example of a json document looks like below. As we can see from the above example, json is very convenient to work with, especially for developers.  But it doesn’t support all the data types available in bson. So, for backup and restoring, we should use binary bson. The second thing to keep in mind is that MongoDB automatically creates databases and collection names if they don’t exist during restore operations. Third, since MongoDB is a document-based database, in many user cases we store large amounts of data in one collection, such as the whole post of an article. MongoDB is also used extensively in large databases and big data. So, reading and inserting the data can consume a lot of CPU, memory and disk space. We should always run the backups during the non-peak hours like night. As already mentioned earlier, we can use import and export functions for backup and restoration of MongoDB databases, but we should use commands like mongodump and mongorestore to backup and restore respectively. MongoDB backup We will first cover backing up the MongoDB database. For this we use the mongodump command.  First open the Windows command prompt and go to the location in which MongoDB is installed. If you have chosen the default setting, while installing MongoDB though the pop-up it will be installed in a location like C:\Program Files\MongoDB\Server\4.4\bin The version number only will change if you are reading this blog in the future. Also, note that it’s better to run the command prompt in the Admin mode. So, once we open the command prompt, we need to change the directory to MongoDB bin folder by giving the below command. cd C:\Program Files\MongoDB\Server\4.4\binNow, enter mongod and press enter. It will show some json text.Now, we can backup to any location. For this post I am backing up on my Desktop in a Backup folder, which I have created through the command line.Now, we have to run mongodump command, but it should be also present in our MongoDB bin folder. If it is not present, we need to download it from and install it. After this, copy the entire exe files from the download to the MongoDB bin folder. MongoDB Backup with no option Now, run the mongodump command from the bin directory. Here, we are not giving any argument so the backup of the whole database will be taken in the same bin directory.MongoDB Backup to an output directory Now, run the mongodump command from the bin directory. Here, the argument –out specifies the directory in which the data backup will be maintained. In our case we are giving the Backup folder in the  Desktop, which we have created earlier. mongodump --out C:\Users\pc\Desktop\Backup Now, go to the desktop and you can find the backup that has been created in our Backup folder.  MongoDB Backup a specific database Now, we can also backup only a database in mongodump using the –db option. I have an example database, so to backup only that I will use the below command. mongodump --db example --out C:\Users\pc\Desktop\Backup As, you can see in the below output only the example database was backed up. MongoDB Backup a specific collection Now, if we want to only backup a specific collection, we need to use the –collection option and give the collection name. Also, note that the database name is mandatory in this case, as mongodb needs to know about the database to search for the collection. I have a products collection within the example database, so to backup only that I will use the below command. mongodump --db example --out C:\Users\pc\Desktop\Backup –collection products As, you can see in the below output only the products collection from example database was backed up. MongoDB Backup from remote MongoDB instances We can get the backup from remote mongodb instances also. I have a lot of MongoDB databases for my personal projects on MongoDB atlas, which is the free to use Cloud database for MongoDB. To get a backup of remote databases, we have to use the connection string with –uri parameter. I used the below command. mongodump --uri "mongodb+srv://xxxx:xxxxxxxxxxx@cluster0.suvl2.mongodb.net/xxxxxDB?retryWrites=true&w=majority" --out C:\Users\pc\Desktop\Backup You can see in the below output the backup of the remote instance. MongoDB Backup procedures We should try to make the backup procedure as automated as possible. One of the best ways is to use a cron job, so that it can run every day. As, discussed earlier it is best to run the backup in the night when the database has the least load.  Setting up a cron job is easier on a Linux or a Mac because the Windows equivalent of it is not good. Alternatively, you can do install mongodb in WSL2 for Windows which supports Ubuntu.  Suppose, on a Linux host which has a mongoDB instance running, you want to run the backup at 04:04 am daily. For this in the terminal, open the cron editor by running the below command in the terminal. sudo crontab –e Now, in the cron editor, you need to add a command like below for our case. 4 4 * * * mongodump --out /var/backups/mongobackups/date +"%m-%d-%y"Restoring and migrating a MongoDB database When we restore the MongoDB database from a backup, we will be able to take the exact copy of the MongoDB information, including the indexes. We restore MongoDB by using the command mongorestore, which works only with the binary backup produced by mongodump. Now, we have taken the backup of example database earlier and it is in our Backup folder. We will use the below command to restore it. In the arguments we will specify the name of the database first with –db option. After that with –drop, we make sure that the example database is first dropped. And in the final argument, we specify the path of our backup. mongorestore --db example --drop C:\Users\pc\Desktop\Backup\example Now, if we check in terminal, we have our example database restored properly. Conclusion In this article, we have learned about MongoDB backup and restore. We have learned the different options for the backups, and why and when backups are required. Keep learning!
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## How to Work With Forms In JavaScript

Forms also referred as web forms are a very important part of front end web application development for sake of interaction with users. Most commonly, forms are used to collect the data from users or provide a provision for user to control the user interface. Forms are great potential assets if correctly used in building an interactive web application. We would be touch basing some of the essential aspects of them like HTML structure, styling form controls, events, data validation and submitting data to server.Understanding forms in detail needs expertise in other areas than just HTML like styling form controls (CSS), scripting to validate or create custom controls (JavaScript).We would be referring or using libraries like Jquery (for document traversal, manipulation etc) and parsley (form validation library) to build better forms.A typical form’s HTML is made of HTML elements called as form controls like single or multiline text fields, dropdowns, checkboxes, button etc mostly created using element with specific type being set on Type attribute. These form controls can be programmed to add some validations to support specific values based on constraints set on them. These controls can be enriched to support accessibility for enabling the interaction for less privileged users.Let’s create a simple html page to build a form.           Learning Forms       All forms have to start with element which is container having the form fields user would interact with. All attributes of element are optional but for programming forms to capture data we need at least ‘action’ and ‘method’ attributes.action – is basically the URL where the form fields data would be sent to.method – corresponds to the HTTP method to submit the form data. Possible HTTP method names which can be set as values are post and get. And another value dialog is set when form is imbedded inside a .Note: Both formaction and formmethod can be overridden by button, input with type submit elements which we will learn as we go forward.Refer to this link to know more about form attributes.Let’s add a form element to our body with action (“”) and method(“get”). This implies that form will send a GET request to the current URL. If it is post then it would be a POST request to the URL in action. Add few fields to form say name, email and a submit button using with type being specified as text, email and submit respectively.Note: The tag is an empty element, meaning that it doesn't need a closing tag. Value attribute can be populated to set the default value.          Enter your name:                    Enter your email:                       Save and open the html in chrome or your preferred browser. Clicking on ‘Click me!’ should send a http get call with empty name and email.Note: We can use instead of with type as submit. The difference is that button can contain HTML content allowing to create a complex button whereas input allows only plain text.Let’s understand the Sending of form data.If we observer all the form fields again, we have added an attribute called ‘name’. This property is important to inform that which data is associated with which form field i.e. name/value pairs. Try adding some data to our fields rendering in html (say myName and first.last@email.com) and click submit button. You should see the data being sent as query parameters in the browser URL.?name=myName&email=first.last@email.com.Change the Form method value to POST instead of GET and send the submitted data by clicking the ‘Click me!’ button. You should be seeing Form Data being sent but the browser URL will not get update.name: myName email: first.last@email.comAll this while, we have our action method being set as empty. Replace this with another URL on server side say ‘/captureFormData’. Now on clicking submit button the data should be received by the script at ‘/captureFormData’ with key/value items contained in the HTTP request object.Note that each server-side language like Node.js, C# etc have their own way of handling the submitted form data. And this blog would not cover those topics and it is beyond the scope.Let’s refine our basic form structure with help of other HTML elements like , , etc. Though we used few of them in basic example. Let’s go little deep on them.Note: Nesting of form inside another form is unacceptable as it might result in unpredictable behavior. is a convenient way of grouping for sake of styling and semantic purpose. This control can be associated with so that some assistive technologies can read this legend and associate it with the controls inside the . Let’s understand this will an example:         Interested programming language                             JavaScript                                     CSharp                                     Java               When reading the above form by any screen readers, it will read as “Interested programming language JavaScript” for the first radio, “Interested programming language CSharp” and “Interested programming language Java” for second and third radio.Imagine if you have a long form with multiple fields. It would help to improve the usability if we can categorize/section them with the help of . It would even help to improve the accessibility of forms.Talking about accessibility, with the associated correctly with the via its for attribute (which contains the element's id attribute), a screenreader will read out something like "name, edit text" for below one.Enter your name: Another advantage of having label associated with input of type text, radio etc is they are clickable too.  If you click on a label then the associated input control will get the focus. If the input control is of type checkbox or radio, clicking on label will select the check box and radio. This will be useful as clickable area of checkbox or radio is small and having label gives provision to select it easily.Note: We can always associate multiple labels to a single input control but it is not a good idea as it will impact the accessibility and assistive technologies. along with can be used to separate the functionality in a form and group the same purpose elements like radio buttons.Here is an example of the same.               Contact information                   Title                                                                             Mr                                                                                                 Mrs                                                                               Name:                                                           E-mail:                                                           Password:                                                 Additional information                               Social type:                                 LinkedIn             Twitter             Instagram                                                 Phone number:                                                           Submit                   Every time you like to create an HTML form you need to start using element and  nesting all the content controls inside it. Most of the assistive technologies and browser plugins can help to discover elements and implement special hooks to make them easier to use.We have already some of the form elements like , , , , , and . Other common input types are button, checkbox, file, hidden, image, password, radio, reset, submit, and text.Input types.Attributes of Input.Few attributes on element help in validating the data like required, max, maxlength, min, minlength, multiple, pattern, step etc based on their respective type.Also other attributes on of type submit/image like formaction, formmethod, formnovalidate, formenctype etc helps in overriding the form level methods.ValidationBefore submitting the data to the server, it is important to perform some client side validation to avoid unwanted round trips. Client-side validation is needed but it is not a replacement to the server side validation. Advantage of having client side validation is to capture the invalid data and fix it immediately.Some of the important and popular checks which are most commonly used on client areField requiredSpecific data formatEnter valid email addressPassword and more…Let’s build a form with the above validation checks.                         Do you have experience in programming ?*                     Yes           No                             How many years of experience you have ?                                     What's your programming language?*                           TypeScript           Java           CSharp           Ruby           Go           Swift                             What's your company e-mail address?                             Cover letter                             Submit       Say, if we enter an value which is more than 40 in experience field. We should see an inbuilt error as shown below:All these validations and notifications are coming out of the box. Thanks to inbuilt functionality in control. Let’s see how we can perform validation of forms using JavaScript and take control of look and feel of error message.Most browsers support constraint validation API by providing few validation properties on HTML elements like , , , etc.validationMessage: we can customize this message if the control value failed validation otherwise it will return an empty string. It is dependent on other constraint i.e. willValidate and isValid.willValidate: If element is validated then it will be true otherwise false.validity: is the validity state of the element and it is dependent on other properties likepatternMatch for specified pattern attribute,tooLong and tooShort are for string fields based on maxLength and minLengthrangeOverflow and rangeUnderflow for numeric fields based on max and min attributestypeMatch for fields which are based on email or url.valid if all the validation constraints are metvalueMissing if the field is set as required.Along with properties, we do also have methods to perform validation like checkValidity() which returns true or false and setCustomValidity(message) is to set the message if the element is considered invalid. Also if the element is invalid then checkValidity will raise an event called invalid Event.Let’s create a simple form and customize the validation message.       Please enter an email address:             Submit     Add a script tag and customize the message as shown below:     const email = document.getElementById("mail");     email.addEventListener("input", function (event) {       if (email.validity.typeMismatch) {         email.setCustomValidity("I am expecting an e-mail address!");       } else {         email.setCustomValidity("");       }     });   Here we are listening to the input event on email field and checking if the validity on the control is valid or not and based on that we are setting the custom message.Here are we relying on inbuilt validation method. Let’s disable the validation at form level by with the help of ‘novalidate’ and take control over validation. This would mean the browser will not perform auto check on validation before sending the data. But still we have access to constraint validation API to perform validation ourself.Refine the above form to add few addition validation like required and minLength etc.               Please enter an email address:                             Submit     Let’s update the script to handle the validation     const form  = document.getElementsByTagName('form')[0];     const email = document.getElementById('mail');     const emailError = document.querySelector('#mail + span.error');     email.addEventListener('input', function (event) {       // Each time the user types something, we check if the form fields are valid.       if (email.validity.valid) {         // In case there is an error message visible, if the field is valid, we remove the error message.         emailError.textContent = ''; // Reset the content of the message         emailError.className = 'error'; // Reset the visual state of the message       } else {         // If there is still an error, show the correct error         showError();       }     });     form.addEventListener('submit', function (event) {       // if the email field is valid, we let the form submit       if(!email.validity.valid) {         // If it isn't, we display an appropriate error message         showError();         // Then we prevent the form from being sent by cancelling the event         event.preventDefault();       }     });     function showError() {       if(email.validity.valueMissing) {         // If the field is empty display the following error message.         emailError.textContent = 'You need to enter an e-mail address.';       } else if(email.validity.typeMismatch) {         // If the field doesn't contain an email address display the following error message.         emailError.textContent = 'Invalid value is entered, expected an e-mail address.';       } else if(email.validity.tooShort) {         // If the data is too short display the following error message.         emailError.textContent = Email should be at least ${ email.minLength } characters; you entered${ email.value.length }.;       }       // Set the styling appropriately       emailError.className = 'error active';     } Reload the HTML and try entering an invalid email address, the corresponding error message should be displayed.Note: In the current scope of this blog, we are not working on styling.Is it possible to validate forms without built in APIs ? Let’s see with the same example.We would consider the same form again but have lot of functionality in                           Please enter an email address:                                             Submit           const form  = document.getElementsByTagName('form')[0];     const email = document.getElementById('mail');     let error = email.nextElementSibling;     const emailRegExp = /^[a-zA-Z0-9.!#$%&'*+/=?^_{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/;     function addEvent(element, event, callback) {       let previousEventCallBack = element["on"+event];       element["on"+event] = function (e) {         const output = callback(e);         // A callback that returns false stops the callback chain and interrupts the execution of the event callback.         if (output === false) return false;         if (typeof previousEventCallBack === 'function') {           output = previousEventCallBack(e);           if(output === false) return false;         }       }     };     // Now we can rebuild our validation constraint. Because we do not rely on CSS pseudo-class, we have to explicitly set the valid/invalid class on our email field     addEvent(window, "load", function () {       // Here, we test if the field is empty (remember, the field is not required)       // If it is not, we check if its content is a well-formed e-mail address.       const test = email.value.length === 0 || emailRegExp.test(email.value);       email.className = test ? "valid" : "invalid";     });     // This defines what happens when the user types in the fiel     addEvent(email, "input", function () {       const test = email.value.length === 0 || emailRegExp.test(email.value);       if (test) {         email.className = "valid";         error.textContent = "";         error.className = "error";       } else {         email.className = "invalid";       }     });     // This defines what happens when the user tries to submit the data     addEvent(form, "submit", function () {       const test = email.value.length === 0 || emailRegExp.test(email.value);       if (!test) {         email.className = "invalid";         error.textContent = "Expecting an e-mail";         error.className = "error active";         return false;       } else {         email.className = "valid";         error.textContent = "";         error.className = "error";       }     });   On refreshing the page, the output with invalid email address should be displayed as shown below.In real time applications, we can rely on existing libraries like Parsley along with JQuery which would ease our life by taking away lot of complexity.Overview of Parsley:Parsley is a front-end javascript validation library which helps to give proper feedback to user on submission of form. As mentioned earlier, it is not a replacement of server side validation. Parsley library helps us to define our own validation.Parsley uses a DOM API namely ‘data-parsley-’ prefix on the existing properties. For example if we want to add this on a property say ‘sample’ then we would add as [data-parsley-sample=’value’]. This will allow us to configure pretty much everything without any configuration or custom function.There is no specific installation process but adding the corresponding script tags will enable the validation. Parsley is relied on Jquery so it has to be included as well.             ...                 $('#form').parsley(); Assumption is that we have downloaded the Jquery and Parsley minified librarie and added it to our working directory. Otherwise we can refer to CDN location as shown below. Adding attribute ‘data-parsley-validate’ to each form will allow us to validate. And “$(‘#form’).parsley()” will manually bind Parsley to your forms.Let’s understand further by configuring the attributes via JavaScript. For which, lets add two input fields inside the form element.                 Also let’s update the content to perform some pre-defined validation based on attributes.       var instance = $('#first').parsley(); console.log(instance.isValid()); // maxlength is 42, so field is valid$('#first').attr('data-parsley-maxlength', 4);       console.log(instance.isValid()); // No longer valid, as maxlength is 4       // You can access and override options in javascript too:       instance.options.maxlength++;       console.log(instance.isValid()); // Back to being valid, as maxlength is 5       // Alternatively, the options can be specified as:       var otherInstance = $('#second').parsley({ maxlength: 10 }); console.log(otherInstance.options); In the console.log, we should see thistrue false true {maxlength: 10}Options are inherited from the global level to form level and further to field. So if we set the options at global level then the same can be observed at field level. Parsley.options.maxlength = 42; // maxlength of 42 is declared at global level var formInstance =$('form').parsley(); var field = $('input').parsley(); console.log(field.options.maxlength); // Shows that maxlength is 42 inherited from global Parsley.options.maxlength = 30; console.log(field.options.maxlength); // Shows that maxlength is automatically 30 formInstance.options.maxlength++; console.log(field.options.maxlength); // Shows that maxlength is automatically 31We can also add our own custom validations. Let understand this with an example. window.Parsley.addValidator('multipleOf', { requirementType: 'integer', validateNumber: function(value, requirement) { return 0 === value % requirement; }, messages: { en: 'This value should be a multiple of %s', } }); Here we are adding a new attribute namely ‘data-parsley-multiple-of’ which takes only numeric values which are multiples of 3.In window.Parsley, we added a new validator with name ‘multiple-of’ with an object containing few important properties like ‘requirementType’, ‘validateNumber’ and ‘messages’ to be shown. This properties helps the library to check if the input value is valid or not.Similar to validateNumber, other properties are also there for different types like validateString, validateDate and validateMultiple.Also for requirementType, we have different options like string, number, date, regexp, boolean etc.Messages by default has English format, to support multiple locales we need to add the specific localization and also add specific locale.Events: Parsley triggers events that allows ParsleyUI to work and for performance reasons they don’t rely on JQuery events but the usage is similar to JQuery i.e. parsley events will also bubble up like JQuery events. For example, if a field is validated then the event ‘field:validate’ will be triggred on the field instance then on to form instance and finally to the window.Parsley.$('#some-input').parsley().on('field:success', function() {         // In here, this` is the parlsey instance of #some-input       });       window.Parsley.on('field:error', function() {         // This global callback will be called for any field that fails validation.         console.log('Validation failed for: ', this.$element); });Many times, we need some validation based on the response from server. Parsley provides an attributes i.e. data-parsley-remote and data-parsley-remote-validator to perform the same.Let’s consider this HTMLLet’s add the async validator on the window.Parsley object.window.Parsley.addAsyncValidator('customValidator', function (xhr) { console.log(this.$element); // jQuery Object[ input[name="q"] ]           return 404 === xhr.status;         }, 'customURL');Parsley is a very useful and powerful JavaScript form frontend validation library.Note: For developers building react based web applications, they can rely on FORMIK which is most popular library for building forms in React and React Native.ConclusionForms are important in HTML and it was needed and still needed now. is an html tag that allow us to perform HTTP methods like GET/POST operation without writing any code in JavaScript. Form defines an boundary to identify all set of the form field elements to be submitted to the server. For example, if we perform an enter key or clicking on submit button , the agent triggers form submission data based on each form field value to the server based on the action URL on the form.Before HTML5, all the elements are expected to be part of the to send the data to server. In HTML5, they maintained the backward compatibility and also enhanced the capabilities who may want to use AJAX and don’t want to rely on default behaviours i.e. they have enabled designers who expect more flexibility in having their form elements outside the form and still maintain the connections with the form.
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How to Work With Forms In JavaScript

Forms also referred as web forms are a very import... Read More