The concept of callbacks and promises can be an intimidating one for many people starting out with Javascript. In essence, callbacks and promises enable asynchronous programming within a Javascript application – allowing developers to write code that gets executed only after a particular event has occurred, often with a delay in between. But up until now, the two approaches have been distinct and separate, creating a difficult decision on which to choose to best fit the requirements of the application. This article will explain both approaches in detail, covering the advantages and disadvantages of each, and also when to use which.

What Is a Javascript Callback?

At its core, a callback is a function that passes a value or parameter to the next function in the chain and then gets executed afterwards. Essentially, when coding callbacks, you are essentially encapsulating logic inside a function and then passing that function as a parameter of another. This allows you to chain together complex logic in an efficient way.

For example, imagine you had two functions that needed to be called one after the other: The first must perform an asynchronous operation and then the second must perform another asynchronous operation. A typical, non-callback solution would be to nest the functions – in this case, you’d need something akin to:

function firstAsyncOperation(params) {	// Some code using params}function secondAsyncOperation(params) {	// Some other code using params	}firstAsyncOperation(params, function() {	secondAsyncOperation(params);});

The snippet above is an example of a typical callback approach. Here, we pass the secondAsyncOperation registered as a callback on the firstAsyncOperation. This approach makes it much easier to code than an alternative nesting approach, such as:

firstAsyncOperation(params, function() {	// Some code using params	secondAsyncOperation(params, function() {		// Some other code using params		});});

Callbacks are a powerful tool for developers, as they allow for complex logic to be written in a concise and efficient manner. Additionally, callbacks can be used to create asynchronous code, which can be beneficial for applications that require a lot of data processing or communication with external services.

What Is a Javascript Promise?

A Javascript promise is an object used when making an asynchronous operation. It can be thought of as an alternative to callbacks that offers several advantages. Its main purpose is to handle notifications between two processes in an asynchronous environment.

At its most basic, a promise resolves a value when called. It is used to wrap an asynchronous action that may or may not return the expected result. When a promise is called, the code is passed the expected value and then the promise executes. In this way, promises allow asynchronous logic to be handled without the use of nested callbacks.

Promises must have one of three states when they finish execution: fulfilled (the action succeeds), rejected (the action fails) or pending (the action is still being processed and no result yet.) Promises also supports the concept of chaining so it is easier for developers to chain together several asynchronous actions into one larger action.

Promises are a great way to handle asynchronous operations in a more organized and efficient manner. They can help reduce the complexity of code and make it easier to debug. Additionally, promises can be used to handle errors in a more graceful way, allowing developers to catch and handle errors in a more controlled manner.

Advantages and Disadvantages of Callbacks

Callbacks offer several advantages, such as their flexibility and how easy they are to use. They also allow for complex logic to be reduced into easily readable functions that can be implemented quickly. But there are drawbacks too – callbacks can get out of hand when nested more than two or three deep, making it harder for developers to manage the flow. Callbacks can also lead to “callback hell” – the scenario where code becomes so unwieldy due to numerous nested callbacks that it becomes hard to modify or debug.

In addition, callbacks can be difficult to debug, as the code is often spread across multiple functions. This can make it difficult to trace the flow of the program and identify the source of any errors. Furthermore, callbacks can be difficult to test, as they are often dependent on external data or functions.

Advantages and Disadvantages of Promises

Promises have some great advantages over callbacks. Their most important advantage is avoiding “callback hell” – since they encapsulate all the logic into one object that can be reused. Additionally, promises are easier to read and make management of multiple asynchronous operations much better. On the other hand, promises can be more complex than callbacks and require more effort to understand and implement correctly.

Difference Between Callback and Promise

The biggest difference between callbacks and promises is that callbacks allow developers to chain together several functions for an asynchronous operation while promises require all actions under one umbrella. Additionally, promises are designed to provide an easier to read and maintain syntax for asynchronous operations.

When to Use Callbacks Versus Promises

When it comes to choosing between callbacks and promises, it is important to understand the advantages and disadvantages of both before making a decision. In general, callbacks are better suited for simple operations while promises should be used for more complex operations or ones with multiple nested operations.

How to Implement Callbacks in Javascript

Callbacks can be implemented in most Node based scripting languages such as Javascript through the use of the then() function. The following is an example of how to create a callback in Javascript:

function doSomethingAsync(foo, bar, callback) {     //Do something asynchronously    callback(value); // invoke the callback passing the value }  doSomethingAsync(foo, bar, function(value) {     // Handle value });

How to Implement Promises in Javascript

Promises can be implemented in Javascript through the use of the Promise constructor. This constructor takes a single parameter – a callback function – which in turn takes two functions as parameters: resolve and reject. Resolve is called if the asynchronous operation succeeds while reject is called if it fails. An example of how to create a promise in Javascript is shown below:

var promise = new Promise(function (resolve, reject) {     // Do something asynchronously     if (/* everything turned out fine */) {         resolve("ok");     } else {         reject(Error("something went wrong"));     } }); 

Best Practices for Using Callbacks and Promises

When implementing callbacks or promises in your code it’s important to follow best practices such as avoiding callback hell (when callbacks get too nested) and always returning a promise when using promises. Additionally, developers should strive to make their code easier to read and maintain by using descriptive names on their callbacks or promises and organizing the code into functions or blocks that have specific tasks and responsibilities.

At the end of the day, callbacks vs promises is a matter of preference and only experience will allow developers to decide which approach works best for their project. This article has provided an overview on each approach as well as detailed best practices on how to implement them safely.