Creating a custom promise in Javascript

26 Oct 2023

promises

Promises in JavaScript are a powerful tool for handling asynchronous operations. While JavaScript provides built-in promises, there may be situations where you need to create a custom promise to handle specific use cases. In this blog post, we will walk you through the process of creating a custom promise in JavaScript.

Understanding Promises
Creating a Custom Promise
Handling Success and Failure
Chaining Promises
Error Handling
Conclusion

Understanding Promises

Promises are objects that represent the eventual completion or failure of an asynchronous operation and allow you to chain multiple asynchronous operations together. Promises provide a clean and readable syntax for handling asynchronous code and can greatly simplify your codebase.

A promise has three states:

Pending: The initial state of a promise.
Fulfilled: The state when the promise is resolved successfully.
Rejected: The state when the promise is rejected or encounters an error.

Creating a Custom Promise

To create a custom promise in JavaScript, you can use the Promise constructor. The Promise constructor takes a callback function with two parameters: resolve and reject. The resolve parameter is called when the promise is successfully fulfilled, while the reject parameter is called when the promise is rejected.

const customPromise = new Promise((resolve, reject) => {
  // Perform asynchronous operation
  // If operation is successful, call resolve(value)
  // If operation encounters an error, call reject(error)
});

Inside the callback function, you can perform your asynchronous operation, such as making an HTTP request or reading a file. Once the operation is complete, you can call either resolve(value) or reject(error) depending on the outcome.

Handling Success and Failure

Once you have created a custom promise, you can handle its success and failure using the then and catch methods. The then method takes a callback function that will be executed when the promise is fulfilled, while the catch method takes a callback function that will be executed when the promise is rejected.

customPromise
  .then((value) => {
    // Handle successful fulfillment
  })
  .catch((error) => {
    // Handle rejection/error
  });

The then method can be chained to handle the success of multiple promises in a sequence. Each then callback function can receive the resolved value from the previous promise.

Chaining Promises

One of the most powerful features of promises is the ability to chain them together. This allows you to create a sequence of asynchronous operations that depend on each other.

customPromise
  .then((value) => {
    // Perform another asynchronous operation
    return anotherPromise;
  })
  .then((value) => {
    // Handle the result of the second promise
  })
  .catch((error) => {
    // Handle any errors in the chain
  });

In the example above, the result of the first promise is passed to the second then callback function, allowing you to perform a dependent asynchronous operation.

Error Handling

Error handling is an important aspect of promises. If any promise in a chain is rejected, the control jumps to the nearest catch handler. This allows you to handle errors in a centralized manner.

customPromise
  .then((value) => {
    // Handle successful fulfillment
  })
  .catch((error) => {
    // Handle rejection/error
  });

By using the catch method, you can handle all errors that occur in the promise chain.

Conclusion

Creating a custom promise in JavaScript gives you more flexibility in handling asynchronous operations. By understanding promises and their states, you can create custom promises and effectively handle both success and failure scenarios. Furthermore, by chaining promises, you can create complex sequences of asynchronous operations. Error handling is also an important part of promises, allowing you to handle errors in a centralized manner. Utilizing these concepts will help you write more efficient and maintainable asynchronous code in JavaScript.

References:

#javascript #promises

Table of Contents