Part one of this article appeared . I covered some interesting features there. :) here Topics I’m gonna cover in this post Promises Async / Await Promises Promises are one of the useful features in ES6. They are used to make operations such as API request, file handling, downloading images, etc. async So, what is async ? ( hold on if you already know) Async operations are operations which take some time to complete. For example, lets say you are defining a function that makes an API request to the server. That function doesn’t return the output immediately. It takes few secs to get the response from the server. So if you are calling that function and assigning it’s value (i.e) output to some variable, it will be Because Javascript doesn’t know that the function is handling some async operations. undefined. So how do we handle it ? Before that let’s talk some history. Before promises, programmers used to define Callbacks are normal functions in Javascript which executes when the async operation is complete. callbacks. For example, you define a function which makes an API request to the server. And then you mention a callback function which will be executed when we get the response from the server. So in the above case, Javascript doesn’t halt the execution till we get the response from the API. And we have defined a function ( callback ) which will be executed after we get the response. I think you got it. So, what are promises ? Promises are objects that helps to do async operations. Technically, they are objects that represents the completion of the async operation. ( If you don’t get it stay with me for a while.) Before explaining how to define a promise I will explain the life cycle of the promise. We have three states in promises : In this state the promise is just executing the async operation. For example, It’s making some API request to the server or downloading some images from cdn. From this state promise can move to either to or to Pending Fulfilled Rejected : If the promise has reached this state, then it means that the async operation is complete and we have the output. For example, we have the response from the API. Fulfilled If the promise has reached this state, it means that the async operation is not successful and we have the error which caused the operation to fail. Rejected: Okay..Let’s see some code. const apiCall = new Promise(function(resolve, reject) {// async operation is defined here...}); Promise is defined by creating a constructor using keyword. Then the constructor will have a function ( we call it .) new executor function The async operation is defined inside the executor function. And note that the executor function have two parameters and . resolve reject The first parameter is actually a function. It it is called inside the executor function and it represents that the async operation is successful and we have got the output. function helps the promise to move from to state. Hope you got it. :) resolve Resolve pending fulfilled Like resolve, is also a function. It is also called inside the executor function and it represents that the async operation is not successful and we have got an error. helps the promise to move from to state. :) reject Reject pending rejected const apiCall = new Promise(function(resolve, reject) {if ( API request to get some data ) {resolve("The request is successful and the response is "+ response);}else {reject("The request is not successful. The error is "+error);}}); In the above code, you can see we have done some async operation inside the executor function. Then the function is called if we get the response from the server. And if there is some error function is called with the error message. resolve reject We are done defining the promise. Let’s see how to execute promise and process the output. // calling the promise.apiCall That’s it. we are done. :) :) Just kidding. It’s not over yet. In the above code, the function is called and the promise is executed (i.e) the function is executed. Then or function is called based on the output. executor resolve reject But you can see that we didn’t handle the output returned from the promise. For example, if we get the response from the API then we have to process the response. Or if we get the error we need to handle it properly. So how do we handle it ? We use to get the output from the promise. handlers Handlers are just functions which executes when some event occurs such as clicking a button, moving the cursor, etc. So we can use handlers to handle when the function is called or function is called. resolve reject Simple. :) Let’s see some code // calling the promise with some handlers.apiCall.then(function(x) {console.log(x); }) In the above code, we have attached a handler to the promise. The handler gets a function parameter. Then the function parameter itself has a parameter . then then x So what’s happening. The handler executes its when the function is called inside the promise. then function parameter resolve I will try to explain it again. The handler looks out for the event that is when the function is called. So when the resolve function is called the handler executes its function parameter. then resolve then apiCall.then(function(x) {console.log(x); }) // OutputThe request is successful and the response is {name: "Jon Snow"} Likewise, there is another handler . catch handler looks out for function. Catch reject function executes its function parameter when the function is called. Catch reject apiCall.then(function(x) {console.log(x); }).catch(function(x) {console.log(x); }) // Assuming the request is not successful ( reject function is called in the promise. ) Output:The request is not successful I think you got it. The above code is not quite readable. So let’s try to refactor it. apiCall.then(function(x) {console.log(x);}).catch(function(x) {console.log(x);}) Ahh… It’s readable now. Most programmers write like this. Ok.. so I think you have come a long way. Let’s have a recap. Promise is defined with keyword with a function parameter. Then the function itself has two function parameters and new resolve reject. Function should be called when the operation is successful. resolve Function should be called when the operation is a failure. reject handler looks out for function. Then resolve handler looks out for function. Catch reject Make sure of the readability of the code :) :) Here’s the working example. Please practice if you are new to this. Promises in Javascript. Hope you understand the example. Its straight forward. Async / Await If you understand Promises, then Async/Await is quite easy. And If you don’t get promises, Async/Await can help you understand it. Maybe you can get a clear escape from promises too. :) Async Async keyword makes any function to return only promises. For example, consider the code below async function hello() {return "Hello Promise..!"} The function will return a promise. hello The above code is equivalent to the below code. function hello() {return new Promise(function(resolve, reject) {// executor function body.});} Simple right? Another Example: async function hello(a, b) {if (a < b) {return "Greater";}else {return new Error("Not Greater");}} hello(14, 10).then(function(x) {console.log("Good..! " + x);}).catch(function(x) {console.log("Oops..! " + x);}) Output:Oops..! Not Greater. // if you call hello(4, 10) you get "Good..! Greater" In the above code, we have defined a function and returned some value or returned some error. async If you are returning some value in the async function , it is equivalent to calling resolve function. If you are returning some error by calling then it is equivalent to reject function. error constructor (i.e) using ‘new’ Don’t forget that async function will return a promise. So of course, you can call and function inside function too. resolve reject async Lets see how that works. async function Max(a, b) {if (a > b) {return Promise.resolve("Success");}else {return Promise.reject("Error");}} Max(4, 10).then(function(x) {console.log("Good " + x);}).catch(function(x) {console.log("Oops " + x);}); Output:Oops Error // If we pass Max(14, 10) then we should get "Good Success" :) Await As the name implies, it makes the Javascript to wait till the operation is done. Let’s say you are making a API request with keyword. It makes the Javascript to wait until you get the response from the endpoint. And then it resumes the execution. await Ok..Let’s go deeper Await can be used only inside async function. It doesn’t work outside async function Let’s see a example async function hello() {let response = await fetch('https://api.github.com/');// above line fetches the response from the given API endpoint.return response;} hello().then(function(x) {console.log(x);});...... Output:Response from the API. In the above code, you can see we have used while fetching response from API. await The fetch operation may take few secs to get the response so till that the execution will be halted and resumes later. Note that await operation only halts the execution inside . The remaining code outside the function will not be affected. The execution continues outside the function. And when we get the response the is executed. hello function hello function parameter inside then handler Hope you got it. Let’s see an example Async/Await in Javascript In the above example, you can see that we have used for function. And the will return an output or an error after 5 seconds. So till that time the execution is halted and then the response is returned. await getResponse getResponse Lets see some real time example. Async/Await in Javascript In the above example, you can see we have used multiple So for each await the execution stops till the response is received and then resumes. awaits. Try the same example with some invalid url. You can see that the error is raised. Error handling is very simple in function. If the error is raised inside the async function or when the error is raised from other functions which is called inside async using then the is called. Simple. async await reject function Hope you enjoyed. I have covered much more interesting topics such as array map, array filter, reduce, etc in the next part. Check it out _ES6 array filter, array map, array reduce, template literals, imports and exports, destructuring objects and arrays…_hackernoon.com ES6 for beginners part-3 If you enjoyed this article try to give some claps and share it :) :)
