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Promises Chaining

Promises/Async/Await: Promises Chaining

What is promise chaining in JavaScript?​

View Answer:
Interview Response: Promise chaining is a technique that allows sequential execution of asynchronous operations by connecting multiple promises, using then() and catch() methods, resulting in cleaner and more maintainable code.

Technical Response: Chaining promises is why we have promises in the first place. It is proper to tell JavaScript the next thing to do after an asynchronous task is done, thus avoiding the pyramid of doom typically associated with nested callbacks. It also reduces the complexity of your code and increases readability.

Code Example:

new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 1000); // (*)
})
  .then(function (result) {
    // (**)

    console.log(result); // 1
    return result * 2;
  })
  .then(function (result) {
    // (***)

    console.log(result); // 2
    return result * 2;
  })
  .then(function (result) {
    console.log(result); // 4
    return result * 2;
  });

How does promise chaining help in error handling?​

View Answer:
Interview Response: Promise chaining centralizes error handling by allowing a single catch() method to handle errors from multiple then() methods, improving readability and reducing the need for multiple error handlers.

Code Example:

function performAsyncTask1() {
    return new Promise((resolve, reject) => {
        // Asynchronous task 1
    });
}

function performAsyncTask2(resultFromTask1) {
    return new Promise((resolve, reject) => {
        // Asynchronous task 2
    });
}

performAsyncTask1()
    .then(resultFromTask1 => performAsyncTask2(resultFromTask1))
    .then(resultFromTask2 => console.log(resultFromTask2))
    .catch(error => console.error('An error occurred:', error)); // error handling in a catch

What is a common mistake made by beginners in JavaScript concerning the chaining of promises?​

View Answer:
Interview Response: A classic mistake made by new developers is breaking the promises chain. New developers often attempt to separate or break the chain for readability or lack of knowledge.

Technical Response: A classic mistake made by new developers is breaking the promises chain. New developers often attempt to separate or break the chain for readability or lack of knowledge. Although technically, we can also add many “.then” to a single promise. This method isn't considered chaining since it adds numerous handlers to a single promise without passing the result. Instead, they process the result independently from one another. We rarely need multiple handlers for one promise in practice, and chaining often gets used.

Code Example:

let promise = new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 1000);
});

promise.then(function (result) {
  console.log(result); // 1
  return result * 2;
});

promise.then(function (result) {
  console.log(result); // 1
  return result * 2;
});

promise.then(function (result) {
  console.log(result); // 1
  return result * 2;
});
note

An example of breaking the chain of Promises is using the promise.then, in an individual invocation, subscribe to a promise.

Can you briefly explain the benefits of returning a new promise in JavaScript?​

View Answer:
Interview Response: Returning a new promise in JavaScript enables proper chaining of asynchronous operations, ensures correct value propagation through the chain, and allows for better error handling and overall code maintainability.

Code Example:

new Promise(function (resolve, reject) {
  setTimeout(() => resolve(1), 3000);
})
  .then(function (result) {
    console.log(result); // 1

    // Returning a Promise
    return new Promise((resolve, reject) => {
      // (*)
      setTimeout(() => resolve(result * 2), 2000);
    });
  })
  .then(function (result) {
    // (**)

    console.log(result); // 2

    return new Promise((resolve, reject) => {
      setTimeout(() => resolve(result * 2), 1000);
    });
  })
  .then(function (result) {
    console.log(result); // 4
  });

Can you briefly explain what a thenable is in JavaScript?​

View Answer:
Interview Response: A thenable in JavaScript is an class, object, or function with a then() method, which can be used in promise chains. Promises are a specific type of thenable, adhering to the Promise/A+ specification.

Technical Response: A “thenable” object is an arbitrary object that has a method .then. It gets treated the same way as a promise. The idea is that 3rd-party libraries may implement “promise-compatible” objects of their own. They can have an extended set of methods and be compatible with native promises, because they implement .then. This feature allows us to integrate custom objects with promise chains without having to inherit from Promise.

Code Example:

class Thenable {
  constructor(num) {
    this.num = num;
  }
  then(resolve, reject) {
    console.log(resolve); // function() { native code }
    // resolve with this.num * 2 after the 1 second
    setTimeout(() => resolve(this.num * 2), 1000); // (**)
  }
}

new Promise((resolve) => resolve(1))
  .then((result) => {
    return new Thenable(result); // (*)
  })
  .then(console.log); // shows 2 after 1000ms

What happens if an error occurs in a chain of promises?​

View Answer:
Interview Response: If an error occurs in a promise chain, the error will be propagated down the chain, skipping remaining then() methods, until it's caught by a catch() method or an unhandled rejection occurs.

Code Example:

firstPromise()
    .then(result1 => secondPromise(result1))
    .then(result2 => thirdPromise(result2))
    .catch(error => console.error('An error occurred:', error));

In this example:

  1. If firstPromise rejects or throws an error, secondPromise and thirdPromise will not be executed. Instead, the control is passed to the catch() method, and the error message from firstPromise will be logged.

  2. If firstPromise resolves but secondPromise rejects or throws an error, thirdPromise will not be executed. Instead, the control is passed to the catch() method, and the error message from secondPromise will be logged.

  3. If both firstPromise and secondPromise resolve but thirdPromise rejects or throws an error, the control is passed to the catch() method, and the error message from thirdPromise will be logged.

In each case, the catch() method handles the error, preventing it from causing a complete halt of the script execution or from resulting in an unhandled promise rejection, which could lead to undefined behavior or application crash. It's also good practice to always have a catch() at the end of your promise chain to ensure that all possible errors are handled appropriately.

How can you return a value from a chained promise in order to use it in subsequent code?​

View Answer:
Interview Response: To use a value from a promise chain in subsequent code, attach a then() method to the end of the chain, and use the value within the callback or return it as another promise.

Code Example:

myPromise
    .then(value => {
        console.log(value);  // Logs: Hello, JavaScript!
        return value + ' How are you?';
    })
    .then(newValue => {
        console.log(newValue);  // Logs: Hello, JavaScript! How are you?
    })
    .catch(error => {
        console.error('An error occurred:', error);
    });

What is the role of the finally() method in promise chaining?​

View Answer:
Interview Response: The finally() method in promise chaining is used to execute code regardless of whether the promises resolved or rejected, making it ideal for cleanup tasks or follow-up actions after the chain.

Can a catch() method be followed by a then() method in promise chaining?​

View Answer:
Interview Response: Yes, a catch() method can be followed by a then() method, allowing you to recover from errors and continue the chain or perform a different action based on the error encountered.

Code Example:

doSomething()
    .then(result => {
        console.log(`Success: ${result}`);
    })
    .catch(error => {
        console.error(`Error: ${error}`);
    })
    .then(() => {
        console.log('This is always called');
    });

How do you handle multiple asynchronous operations with different error-handling using promise chaining?​

View Answer:
Interview Response: To handle different error handling in a promise chain, you can place catch() methods after each then() method, allowing you to specifically handle errors related to each individual asynchronous operation.

Code Example:

firstPromise()
    .then(result1 => {
        // Use result1
        return secondPromise(result1);
    })
    .catch(error => {
        console.error('An error occurred in firstPromise:', error);
    })
    .then(result2 => {
        // Use result2
        return thirdPromise(result2);
    })
    .catch(error => {
        console.error('An error occurred in secondPromise:', error);
    })
    .then(result3 => {
        // Use result3
    })
    .catch(error => {
        console.error('An error occurred in thirdPromise:', error);
    });

In this example:

  1. If firstPromise fails, its error will be caught in the first .catch() block and the subsequent .then() and .catch() blocks will not be executed.

  2. If secondPromise fails, its error will be caught in the second .catch() block and the last .then() and .catch() block will not be executed.

  3. If thirdPromise fails, its error will be caught in the last .catch() block.

Keep in mind that in this structure, if a promise fails, the following promises will not be executed because a rejected promise will immediately lead the control flow to the nearest .catch() block.

This method provides granular control over error handling for each promise, allowing you to handle each error in a different manner if needed.

note

This way of handling errors in promises is not very common because usually we want to execute all asynchronous tasks and handle all the errors in a centralized .catch() block. But it can be useful in some specific cases.

Can you describe how promise chaining helps in avoiding callback hell?​

View Answer:
Interview Response: Promise chaining avoids callback hell by providing a flat, linear structure for handling asynchronous operations, reducing nested callbacks and improving readability and maintainability of the code.

How do you transform callback-based functions into promises for chaining?​

View Answer:
Interview Response: You can transform callback-based functions into promises using the Promise constructor, by placing the callback-based function inside the constructor and resolving or rejecting the promise in the callback.

Code Example:

function callbackFunction(data, callback) {
    // ... some async operation
    callback(error, result);
}

// transforming the above function into a promise-based function
function promiseFunction(data) {
    return new Promise((resolve, reject) => {
        callbackFunction(data, (error, result) => {
            if (error) {
                reject(error);
            } else {
                resolve(result);
            }
        });
    });
}