A Promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation. It serves as a placeholder into which the successful result value or reason for failure will materialize.
To create a promise in JavaScript, you can use the Promise constructor which takes in a function as a parameter. This function takes two parameters: a resolve function (if the promise is fulfilled) and a reject function (if the promise is rejected).
```
let promise = new Promise(function(resolve, reject) {
// some asynchronous operation
});
```
Here are the steps on how to use promises:
1. Creating a Promise:
```
let promise = new Promise(function(resolve, reject) {
// Asynchronous operation here
1. Using the Promise:
Promises have two methods: `.then()` and `.catch()`. `.then()` has two callback parameters, first one is invoked when the promise is resolved and the second one (optional) when the promise is rejected. `.catch()` is used if promise is rejected and no reject callback is provided in `.then()`.
```
promise
.then(function(result) {
console.log(result); // Will print ‘Result’ if promise is resolved
}, function(error) {
console.log(error); // Will print ‘Error’ if promise is rejected
});
// OR
promise
.then(function(result) {
console.log(result); // Will print ‘Result’ if promise is resolved
})
.catch(function(error) {
console.log(error); // Will print ‘Error’ if promise is rejected
});
```
1. Chaining Promises:
Promises can be chained. This means that you can attach a `.then()` to the previous `.then()` and create a chain of promises. The value returned from a `.then()` callback function is passed as an argument to the next `.then()`.
```
doSomething()
.then(result => doSomethingElse(result))
.then(newResult => doThirdThing(newResult))
.then(finalResult => {
console.log(‘Got the final result: ‘ + finalResult);
})
.catch(failureCallback);
```
In this example, `doSomething`, `doSomethingElse` and `doThirdThing` are functions that return Promises. They are executed one after the other, because each `then` waits for the previous Promise to resolve or reject. If any Promise in the chain rejects, the `catch` is called.
