What are the pros and cons of using Promises instead of callbacks in JavaScript?

Answer

1. Improved Readability

• Promises avoid "callback hell" by chaining .then() and .catch() instead of deeply nested callback functions.

• Example:

  // Using Promises
  fetchData()
    .then((data) => process(data))
    .then((result) => console.log(result))
    .catch((error) => console.error(error));
  
  // Using Callbacks
  fetchData((error, data) => {
    if (error) {
      console.error(error);
    } else {
      process(data, (error, result) => {
        if (error) {
          console.error(error);
        } else {
          console.log(result);
        }
      });
    }
  });

2. Built-in Error Handling

• Promises propagate errors automatically through the .catch() method, simplifying error handling.

3. Chaining

• Promises allow sequential execution of asynchronous operations with .then() chaining, making the flow easier to follow.

4. Composability

• Promises can be combined using Promise.all(), Promise.race(), and other utilities for handling multiple asynchronous operations efficiently.

  Promise.all([fetchData(), fetchUser(), fetchSettings()])
    .then(([data, user, settings]) => {
      console.log(data, user, settings);
    })
    .catch((error) => console.error(error));

5. Integration with Modern Features

• Promises work seamlessly with async/await, making asynchronous code even more readable and synchronous-looking.

Cons of Using Promises

1. Complexity for Simple Tasks

• For very simple asynchronous operations, Promises can add unnecessary complexity compared to a straightforward callback.

  // Callback
  setTimeout(() => console.log('Done!'), 1000);
  
  // Promise
  new Promise((resolve) => setTimeout(resolve, 1000)).then(() =>
    console.log('Done!')
  );

2. Increased Learning Curve

• Promises require understanding of chaining, error propagation, and utilities like Promise.all().

3. Potential for Silent Failures

• If a .catch() or error handler is not attached, errors might silently fail or remain unhandled.

4. Backward Compatibility Issues

• Older JavaScript codebases using callbacks may require refactoring to adopt Promises.

When to Use Promises

• When dealing with complex asynchronous operations requiring multiple steps or error handling.
• When chaining or composability is essential.
• When integrating with modern JavaScript (async/await).

When to Use Callbacks

• For simple, one-off asynchronous tasks.
• In performance-critical code where Promises might introduce slight overhead.

MDN Web Docs: Promises