Promisification

Promises/Async/Await: Promisification

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What is Promisification in JavaScript?

View Answer:

Interview Response: Promisification in JavaScript is the process of converting callback-based functions into Promise-based functions, enabling easier handling of asynchronous operations and improved code flow management.

Technical Response: “Promisification” is a long word for a simple transformation, and it is the conversion of a function that accepts a callback into a function that returns a promise. Such transformations are often required in real-life, as many functions and libraries are callback-based. But promises are more convenient, so it makes sense to “promisify” them.

Code Example:

let loadScriptPromise = function (src) {
  return new Promise((resolve, reject) => {
    loadScript(src, (err, script) => {
      if (err) reject(err);
      else resolve(script);
    });
  });
};

// usage:
// loadScriptPromise('path/script.js').then(...)

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Why do we need Promisification?

View Answer:

Interview Response: Promisification simplifies asynchronous code, helps avoid callback hell, and allows for better error handling and chaining of asynchronous operations using Promise chaining or async/await.

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What is a Promise in JavaScript?

View Answer:

Interview Response: A Promise is an object representing the eventual completion or failure of an asynchronous operation, allowing developers to attach callbacks to handle results or errors.

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How does Promisification relate to async/await?

View Answer:

Interview Response: Promisification converts callback-based functions into ones returning Promises. Async/await, syntactic sugar for Promises, allows asynchronous code to appear synchronous, improving readability and error handling when using Promisified functions.

Code Example:

Now let's consider a simple callback-based function that we will then promisify and use with async/await.

// Original callback-based function
function getDataCallback(id, callback) {
  setTimeout(() => {
    if (id > 0) {
      callback(null, { id: id, name: 'Name'+id });
    } else {
      callback('Invalid ID');
    }
  }, 2000);
}

// Promisification
function getDataPromise(id) {
  return new Promise((resolve, reject) => {
    getDataCallback(id, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

// Using async/await
async function printData(id) {
  try {
    const data = await getDataPromise(id);
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

printData(1); // Output after 2 sec: { id: 1, name: 'Name1' }

In this example, the function getDataCallback uses the callback pattern. We've created getDataPromise to promisify getDataCallback. Then, we use the async/await syntax in printData to consume getDataPromise, which allows for easier-to-read, synchronous-style code.

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What is the purpose of the resolve and reject functions in a Promise constructor?

View Answer:

Interview Response: In a Promise constructor, `resolve` and `reject` are functions used to signal the Promise's outcome. `resolve` fulfills the promise with a value, while `reject` indicates the promise failed with a reason.

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How does error propagation work in Promisification?

View Answer:

Interview Response: In JavaScript, error propagation in Promises works through the `reject` function provided in the Promise constructor. When an error occurs, you call `reject` with the error, which can then be caught by attaching a `.catch` block to the Promise.

Code Example:

// Promisified function
function riskyOperation() {
    return new Promise((resolve, reject) => {
        let operationSuccessful = Math.random() > 0.5; // simulate a risky operation

        if (operationSuccessful) {
            resolve('Operation succeeded!');
        } else {
            reject(new Error('Operation failed!'));
        }
    });
}

// Using the function with error handling
riskyOperation()
    .then(result => {
        console.log(result); // this will be called if operation is successful
    })
    .catch(error => {
        console.error('Error occurred:', error.message); // this will be called if operation fails
    });

In this example, if operationSuccessful is false, we call reject with a new Error object. This error can then be caught by attaching a .catch block to the Promise. If an error is thrown in the function passed to new Promise (even outside the reject call), it will also be propagated as a rejection.

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What is a common pattern for Promisifying a function with a single callback?

View Answer:

Interview Response: A common pattern is to create a function that returns a new Promise. The executor function of the Promise wraps the original callback-based function and calls `resolve` or `reject` in the callback.

Code Example:

function promisifiedFunction(params) {
  return new Promise((resolve, reject) => {
    originalFunction(params, (error, result) => {
      if (error) {
        reject(error);
      } else {
        resolve(result);
      }
    });
  });
}

This promisified function can now be used with .then(), .catch(), or async/await.

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How does Promise.all() help in Promisification?

View Answer:

Interview Response: `Promise.all()` allows you to aggregate several promises and waits for all of them to complete. It's very useful in the context of promisification when you have multiple async operations that need to be run in parallel.

Code Example:

// Assume these are promisified functions that return promises
function fetchData1() {
  return new Promise(resolve => {
    setTimeout(() => resolve('Data 1'), 1000);
  });
}

function fetchData2() {
  return new Promise(resolve => {
    setTimeout(() => resolve('Data 2'), 2000);
  });
}

function fetchData3() {
  return new Promise(resolve => {
    setTimeout(() => resolve('Data 3'), 1500);
  });
}

// Use Promise.all() to wait for all of them
Promise.all([fetchData1(), fetchData2(), fetchData3()])
  .then(values => {
    console.log(values);  // ["Data 1", "Data 2", "Data 3"]
  })
  .catch(error => {
    console.error(error);
  });

In this example, Promise.all() is used to wait for all fetchData promises to resolve. The results are then logged in the order the promises were in the array, not the order in which they resolved. If any promise is rejected, Promise.all() immediately rejects with the reason of the first promise that rejected, discarding all the other promises whether or not they have completed.

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What is the role of Promise.prototype.finally() in Promisification?

View Answer:

Interview Response: `Promise.prototype.finally()` is a method on the Promise prototype that allows you to specify logic that gets run after the Promise has been settled, whether it was fulfilled or rejected. This is useful in promisification when you have some logic that needs to run regardless of the promise outcome.

Code Example:

// Assume this is a promisified function
function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const success = Math.random() > 0.5; // random success or failure
      if (success) {
        resolve('Data fetched');
      } else {
        reject(new Error('Fetch failed'));
      }
    }, 2000);
  });
}

// Use the fetchData promise
fetchData()
  .then(data => {
    console.log(data); // this will run if the promise is resolved
  })
  .catch(error => {
    console.error(error.message); // this will run if the promise is rejected
  })
  .finally(() => {
    console.log('Operation attempted.'); // this will always run
  });

In this example, regardless of whether fetchData is fulfilled or rejected, the finally block will execute, logging 'Operation attempted.' This can be very useful for cleanup tasks like closing database connections, stopping loading spinners, etc.

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Can you Promisify functions with multiple callbacks?

View Answer:

Interview Response: Yes, you can promisify functions with multiple callbacks, but it may be slightly more complex because promises inherently only resolve or reject once. If your function has multiple callbacks that are all called only once, you can return an object or array of promises.

Code Example:

// Original function
function originalFunc(success1, success2, error) {
  setTimeout(() => {
    if (Math.random() > 0.2) { // 80% chance of success
      success1('Success 1 result');
      success2('Success 2 result');
    } else {
      error('Error result');
    }
  }, 1000);
}

// Promisified function
function promisifiedFunc() {
  return new Promise((resolve, reject) => {
    originalFunc(
      result1 => resolve({result1}),
      result2 => resolve({result2}),
      error => reject(error)
    );
  });
}

// Usage
promisifiedFunc()
  .then(result => console.log(result))
  .catch(error => console.error(error));

This example wraps originalFunc in a promise. If success1 or success2 are called, the promise is resolved with the respective result. If error is called, the promise is rejected with the error. Note that once the promise is either resolved or rejected, subsequent calls to resolve or reject will have no effect, which is why this approach only works if each callback is expected to be called once.

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danger

If your function calls a callback multiple times, promises may not be the best solution, as they can only be resolved or rejected once. For such situations, EventEmitters or Observables (like those from RxJS) might be a better fit.

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How does Promisification improve error handling?

View Answer:

Interview Response: Promisification can significantly improve error handling in asynchronous JavaScript code by enabling you to use `.catch()` blocks and `try-catch` structures (with async/await) to handle errors in a linear, more readable way. This is a big improvement over callback-based error handling which can lead to deeply nested structures.

Code Example:

Consider this callback-based function:

// Original callback-based function
function getDataCallback(id, callback) {
  setTimeout(() => {
    if (id > 0) {
      callback(null, { id: id, name: 'Name'+id });
    } else {
      callback('Invalid ID');
    }
  }, 2000);
}

// Usage
getDataCallback(0, (err, data) => {
  if (err) {
    console.error(err);
  } else {
    console.log(data);
  }
});

Now, let's promisify it and handle errors:

// Promisification
function getDataPromise(id) {
  return new Promise((resolve, reject) => {
    getDataCallback(id, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

// Usage with .catch()
getDataPromise(0)
  .then(data => console.log(data))
  .catch(err => console.error(err));

// Usage with async/await
async function printData(id) {
  try {
    const data = await getDataPromise(id);
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

printData(0);

In both usages of getDataPromise, we have a clear, linear flow of error handling that's easy to read and manage. In the .catch() example, any error in the Promise chain is handled in the .catch() block. With async/await, we can use try-catch just like with synchronous code.

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How do you handle timeouts in Promisified functions?

View Answer:

Interview Response: When dealing with Promises, you can create a utility function that returns a Promise that rejects after a certain timeout. This can then be used with `Promise.race()` to implement a timeout for a Promise.

Code Example:

// Utility function for creating a promise that rejects after a timeout
function timeout(ms) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            reject(new Error(`Operation timed out after ${ms} ms`));
        }, ms);
    });
}

// Assume this is a promisified function
function fetchData() {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve("Data fetched");
        }, 5000); // This operation takes 5 seconds to complete
    });
}

// Use Promise.race to implement a timeout
Promise.race([fetchData(), timeout(3000)])
    .then(data => {
        console.log(data); // This will run if fetchData completes before the timeout
    })
    .catch(error => {
        console.error(error.message); // This will run if fetchData does not complete before the timeout
    });

In this example, Promise.race() is used to start fetchData() and timeout(3000) in parallel. Whichever promise settles first will win the race, and its result (or error) will be passed to the following .then() (or .catch()). If fetchData() takes longer than 3000 milliseconds, the timeout promise will reject, causing the error to be logged.

W

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What is one crucial aspect to remember when using promisifications?

View Answer:

Interview Response: Promisification is an excellent approach, especially when using async/await, but not a total replacement for callbacks. Remember, a promise may have only one result, but a callback may technically be called many times. So promisification is only meant for functions that call the callback once, and further calls get ignored.

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