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Javascript Promise W3schools: Javascript Explained

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Javascript promises are a powerful tool for controlling the flow of async operations in your code. As most Javascript operations either often have to rely on network requests like database queries, they often have to be asynchronous. With promises, developers can control a set of asynchronous tasks which need to be run in order, including providing a way to handle errors. In this article, we will explore what promises are, how to use them, and some tips and tricks for working with them.

What is a Javascript Promise?

A promise is a special JavaScript object that represents the eventual completion or failure of an asynchronous operation. It allows you to write code that’s more predictable and easier to debug. The basic syntax for creating a promise is as follows:

let promise = new Promise(function(resolve, reject) {  // do something (usually asynchronous)  // then call resolve(...) when everything is ok  // or reject(...) when something goes wrong});

Promises allow developers to control a set of asynchronous tasks, which are passed in as an argument to the promise constructor. The promises API includes a few states which can be used to understand and track the progress of the asynchronous task. Each of these states has a few corresponding methods that you can use to read the state or do something when the state changes.

  • Pending: The initial state of a promise, when it has not been fulfilled or rejected.
  • Fulfilled: The state of a promise when it has completed successfully.
  • Rejected: The state of a promise when it has failed.

Promises are a great way to handle asynchronous operations in JavaScript, as they provide a simple and predictable way to handle errors and results. Additionally, promises can be chained together, allowing developers to create complex asynchronous operations that can be easily debugged and tracked.

How to Create a Javascript Promise

Creating a promise is easy and straightforward. All you need to do is call the Promise constructor and pass in a callback function. The callback function will contain the logic of the asynchronous operation, and should have two arguments: resolve and reject. These arguments should be called when the promise’s result is available.

The “resolve” call should be used if the operation was successful. This will trigger the promise to go into the “fulfilled” state. The “reject” call should be used if the operation failed. This will trigger the promise to go into the “rejected” state.

let promise = new Promise(function(resolve, reject) {  // perform some asynchronous task  if (taskWasSuccessful) {    // call resolve if successful    resolve();  } else {    // call reject if unsuccessful    reject();  }});

Once the promise is created, you can use the then() method to register callbacks for when the promise is fulfilled or rejected. The then() method takes two arguments: a callback for when the promise is fulfilled, and a callback for when the promise is rejected. These callbacks will be called with the value that was passed to the resolve or reject call.

Understanding the Promise States

As mentioned earlier, promises have several states that developers can use to understand the progress of their asynchronous operations. Depending on the state, it is possible to add callback functions to execute when a specific state is reached. This can be done using the following methods:

  • .then(): Call this method to add a callback that will be triggered when the promise goes into the “fulfilled” state.
  • .catch(): Call this method to add a callback that will be triggered when the promise goes into the “rejected” state.
  • .finally(): Call this method to add a callback that will be triggered when either state of the promise is reached (fulfilled or rejected).
  • Promise.all() is used when you have an iterable (like an array) of promises and you want to wait for all of them to be fulfilled, or for any of them to be rejected.
  • Promise.race() accepts an iterable of promises and returns a promise that fulfills or rejects as soon as one of the promises in the iterable fulfills or rejects, with the value or reason from that promise
  • Promise.allSettled() accepts an iterable of promises and returns a promise that resolves after all of the given promises have either fulfilled or rejected, with an array of objects that describe the outcome of each promise

These methods allow developers to write code that is executed when either state of the promise is reached. This can be useful for controlling asynchronous flows and handling errors in an organized manner.

It is important to note that promises are not limited to the three states mentioned above. Developers can also create custom states to better suit their needs. Additionally, promises can be chained together to create complex asynchronous flows that can be managed in a more efficient manner.

Benefits of Using Promises

Using promises has several benefits, both in terms of performance and developer experience. By using promises, developers can write code that is more organized and easy to debug. Furthermore, promises can increase performance by providing a mechanism for handling asynchronous tasks in an organized manner. Finally, promises provide a way for developers to write code that is future-proof, as promises are part of the ES2015 standard.

Promises also provide a way for developers to write code that is more maintainable. By using promises, developers can break down complex tasks into smaller, more manageable chunks. This makes it easier to debug and modify code, as well as to add new features. Additionally, promises can help developers avoid callback hell, which is a common issue when dealing with asynchronous tasks.

Examples of Javascript Promises

Here are a few examples of how promises can be used in your code. All of these examples assume you have already created a promise object.

promise.then(function() {  // code to be executed when the promise is fulfilled (i.e., the asynchronous task has finished).}); promise.catch(function() {   // code to be executed if the promise is rejected (i.e., if the asynchronous task has failed).  });  promise.finally(function() {   // code to be executed when either state of the promise is reached (fulfilled or rejected).  }); 

Example Code Chaining Promises

Imagine you need to fetch user data from an API and then, based on the user’s ID, fetch their orders. This is a two-step asynchronous process where the second step depends on the outcome of the first.

fetchUserData().then((userData) => {
  return fetchUserOrders(userData.id);
}).then((orders) => {
  console.log(orders);
}).catch((error) => {
  console.error("An error occurred:", error);
});

In this example, fetchUserData() and fetchUserOrders(id) are functions that return promises. The orders are fetched only after the user data has been successfully retrieved, demonstrating the sequential execution of asynchronous tasks using promise chaining. If any promise in the chain is rejected, the error will be caught by the .catch() method.

Tips for Working With Promises

When working with promises, there are a few tips that can help you get the most out of them. Firstly, make sure you test your promises thoroughly by writing code to test all possible cases (success, failure and error). Secondly, use descriptive names for your promises to make debugging easier. Finally, don’t forget to call the reject callback if there are any errors in your code! This will ensure that any errors are handled properly.

Common Pitfalls of Javascript Promises

While promises are a powerful and useful tool, they do come with some pitfalls. Firstly, promises can be difficult to debug if you aren’t familiar with how they work. Secondly, since they’re asynchronous they can often cause race conditions, which can be difficult to isolate and fix. Finally, promises are not always backwards-compatible, so make sure you check your code before using a new feature.

Troubleshooting Common Issues With Javascript Promises

Working with promises can sometimes lead to unexpected issues. To troubleshoot these, it’s important to understand how promises work and which methods are available for debugging them. Start by checking your code for any mistakes or typos that could be causing the issue. If this doesn’t work, try using console.log statements or debugging tools like Chrome DevTools to help you track down and fix any issues.

Conclusion

Javascript promises are a powerful tool for controlling async operations in your code. They provide a way to write code that’s more predictable and easier to debug by allowing developers to control a set of asynchronous tasks which need to be run in order. Finally, they come with several useful methods that can be used to track and debug them if any issues arise.

Nisha Kumari

Nisha Kumari

Nisha Kumari, a Founding Engineer at Bito, brings a comprehensive background in software engineering, specializing in Java/J2EE, PHP, HTML, CSS, JavaScript, and web development. Her career highlights include significant roles at Accenture, where she led end-to-end project deliveries and application maintenance, and at PubMatic, where she honed her skills in online advertising and optimization. Nisha's expertise spans across SAP HANA development, project management, and technical specification, making her a versatile and skilled contributor to the tech industry.

Written by developers for developers

This article was handcrafted with by the Bito team.

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