Promise.prototype.then() : JavaScript then() method and how to use it
Javascript , Requests , Uncategorized / September 25, 2019

Q:  How to use then() and how to return a promise from a function and use then() ?   A: then() is a method in the Promise prototype that returns a promise and accepts two callback functions. One function is for the success and the other is for the failure case of the promise. If both arguments are not given or are not callback functions, still then the method would not generate any errors. For example, here we create a new promise.

promise .then() exists hence it is a promise. .then() allows method chaining.   Now we will see how to use then to return a promise It is incorrect to use then() this way as it should return inside the then also. Otherwise resolving the promise returned by then will give undefined.

    It is incorrect to use the function without returning the promise returned  by then()

    Returning the promise returned by then is the correct way of returning a promise from a function

    If there is some kind of logic to be applied, this is the correct way of using then()

 

Understanding Promises in JavaScript : asynchronous behaviour of Javascript Part -2

In the previous article, we learned about using callback for performing asynchronous operations. In ES06/ES2015, Javascript introduced something called Promises which helps you handle  asynchronous operations in a better way The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a better way for managing asynchronous operations when compared to traditional callback-based way. Promises represent data that may not be available but will be available later upon completing some asynchronous operation. A promise can be called a placeholder for future data and  has 3 states PENDING RESOLVED REJECTED Immediately on creation, the state of a Promise is pending. Resolved state means the asynchronous operation is fulfilled. Rejected means that the asynchronous operation is not completed and has some errors. Many Javascript native apis for asynchronous operations support promises since ES06. Prior to that many libraries including JQuery and Bluebird were used to create promises for doing asynchronous operations. Example for promises JavaScript fetch method returns a promise.

You can see that immediately the state of promise is pending. Now for resolving the promise and getting the output Promise.prototype.then() and Promise.prototype.catch() Promise.prototype.then() and Promise.prototype.catch() The promise object has a then method that returns the promise…

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