JavaScript Promises from Scratch

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To get the most out of this article, please make sure that you know basics about Synchronous and Asynchronous Programming and, if possible, about javascript callbacks.However, I will try to keep most of the stuff away from these topics so that you can understand at least 85% of it. Understanding Promises Suppose a friend comes to you for financial help and you promise him to give some cash after taking it out from an ATM. You go to the ATM, swipe your card, and find out that your account has been frozen due to some reasons and you can’t take out your money. Then, you go back to your friend and tell him that you can’t give him money because your account is frozen. Hence, breaking your promise. //You have made promise to your friend and you leave for bank if (everythingGoesWell) { return theMoney //Promise is fulfilled } else { return “Account_Frozen” //Promise failed } Promises are used in JavaScript to handle async operations. They are special objects that link the actual output (in the above example, it’s required money) and the reason due to which we may not get the output (Account Frozen). States of a Promise A JavaScript promise is either settled or pending . We can use the Promise constructor to create a promise . var thePromise = new Promise(executor()) The executor function is a callback of the promise constructor. It takes two callbacks:  resolve  and  reject , as arguments. The  resolve  callback is used when the promise is actually fulfilled. It takes the output value as its argument. The  reject  callback is used when the promise couldn’t be fulfilled. It takes the reason as its argument. When we create a promise it initializes in its  pending  state. Once the executor runs, the state changes to  settled  which can be either resolved  or  rejected . Implementation: Code Snippet var thePromise = new Promise(function(resolve,reject){ // Try to take out money from atm withdraw(function(error,money) { // If withdrawal failed, tell why if (error) reject (error) // else return the money else resolve (money) }) }) Promises can be a bit of hectic stuff if you don’t have experience with callbacks and callback hells. But once you grasp this concept, there is no going back! The  then  and  catch  clauses The  catch  function is attached to promise which executes when the promise is rejected. It takes the error sent through  reject  method as an argument. The  then  function is attached to promise which executes when the promise is resolved. It sends the value sent through resolve method as an argument. There is a  finally  clause too. It is executed no matter the promise resolves or rejects. It takes no arguments. var thePromise = new Promise(function(resolve,reject){ // Try to take out money from atm withdraw(function(error,money) { // If withdrawal failed, tell why if (error) reject (error) // else return the money else resolve (money) }) }) thePromise.then(function(money) { // This function is executed when the above promise resolves and returns an //amount of money }) thePromise.catch(function(error) { // This function is executed when the above promise rejects due to an error }) thePromise.finally(function() { // This function is executed after the promise is resolved or rejected.  » Read More

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