Difference Between Callbacks, Async/Await and Promise in React Native

In this blog, we will see fundamental concepts like Callback functions, promises, and the use of async and await to handle different operations in React Native.

Before we start with the difference, let’s understand what is synchronous and asynchronous.

Difference Between Synchronous and Asynchronous

Let’s take a live example to understand asynchronous and synchronous differences. Suppose you go to a restaurant and a waiter comes to your table, takes the order, and gives it to the chef. Then he moves to another table to take the order while the chef is preparing your meal. This means the same waiter can serve many different tables. This is called asynchronous means a non-blocking way of working. Here we can say waiter is a single thread allocated to handle multiple requests.     

Synchronous is a contrast to asynchronous architecture. Let’s see how it works with the same restaurant example. Suppose you visit another restaurant where a waiter is only allocated to you. He will take your order and give it to the chef. Now, this waiter will wait until the chef prepares your meal and during this time he is not doing anything but just waiting for your meal to get ready. This is known as Synchronous means blocking the way of working.

Single-threaded events are handled using an event or a message queue. So when a code is being executed in a synchronous way the thread will wait until the first statement is done and then jump to the second one, while asynchronous execution will be continued even if the first one was not completed.

We can handle this asynchronous code by the use of callbacks, promises and async/await.

Callbacks in React

In JavaScript, functions are objects which allow passing objects as parameters to other functions. Callbacks are nothing but passing functions as parameters to other functions and calling them inside the function. It will run synchronously, the second function will run once the first is finished.

Example of callbacks in react:

Synchronously the code would have given the output as,

Tom

Jack

Jerry

But we have used setTimeout (calls function after a given period of time in milliseconds) which is an asynchronous function that will show output as,

Tom

Jerry

Jack

The problem with callback functions is that it nest functions within functions and it can go on. This gets really hard to understand and read the code.

Promise in React

A promise is an assurance that we will do something in the future, the same as real-life promises. These promises will either be kept when the time comes or they won’t. Similarly, this works in React Native too. When we make a promise in React Native, it will be executed when the execution time comes, or it will be rejected. 

A promise is used to handle the asynchronous output of an executed operation. With promises, we can execute a code block until a non-block async request is complete. With this other operations can be kept running without any interruption.

States of executing promise in react

• Pending state when promises are in progress and not yet failed or succeeded
• Success state when the promise is successfully executed
• Failure state when the promise is failed and throws an error

Using Promise:

To use promises we have used then() which is called when a promise is resolved and catch() is called when the promise is rejected. catch() will catch an error while then() will pass the response.

Chaining in Promises

Calling asynchronous requests multiple times is called Chaining. When the first promise is resolved or rejected, the second promise will start.

Async/Await In React

Await is basically syntactic sugar for Promises. It makes your asynchronous code look more like synchronous code, which is easier to understand.

Syntax of Async and Await:

Wrapping the function with “async” lets JS know that we are using async/await. The use of “async” is necessary when we want to use await which means we can’t use Await at the global level. The “await” keyword is used to ensure that all promises are synchronized which means they wait for each other to get executed. With waiting we can eliminate the use of callbacks. Async returns a promise and awaits prepend while calling a promise.

Example to understand async/await: