Contents
A promise is an object that represents the result of an asynchronous operation. Promises are used in JavaScript to handle asynchronous tasks in a more synchronous way. When an asynchronous task is started, a promise is created. The promise is then resolved or rejected when the task is finished.
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Introduction
A promise is an object that may produce a single value some time in the future : either a resolved value, or a reason that it’s not resolved (e.g., a network error). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.
Creating a promise
A Promise is created using the new keyword :
let promise = new Promise(function(resolve, reject) {
// do a thing, possibly async, then…
if (/* everything turned out fine */) {
resolve(“Stuff worked!”);
}
else {
reject(Error(“It broke”));
}
});
What is a Promise?
A promise is an object that represents the result of an asynchronous operation. A promise can be in one of three states:
– pending: The async operation has not yet completed.
– fulfilled: The async operation completed successfully.
– rejected: The async operation failed.
When a promise is pending, it means that the async operation is still in progress and has not yet resolved. Once the async operation completes, the promise will be either fulfilled or rejected. If the async operation was successful, the promise will be fulfilled with a value. If the async operation failed, the promise will be rejected with an error.
Promises are used in many modern JavaScript libraries and frameworks, such as AngularJS, ReactJS, Node.js, and Vue.js.
The Promise Interface
A promise is an interface for representing an asynchronous operation. It provides a way to register callbacks for when the operation completes, and allows you to chain together multiple operations in a way that is convenient and easy to read.
There are three states that a promise can be in:
Pending: The initial state of a promise. The operation has not yet completed.
Fulfilled: The operation has completed successfully.
Rejected: The operation has failed.
A promise can be rejected for any reason, including but not limited to:
-The asynchronous operation failed.
-The data that was returned from the asynchronous operation was invalid.
-The user cancelled the asynchronous operation.
Creating a Promise
In order to create a Promise, you need to use the `Promise` constructor as follows:
“`javascript
const myPromise = new Promise((resolve, reject) => {
// do something asynchronous which eventually calls either:
// resolve(someValue); // fulfilled
// or
// reject(“failure reason”); // rejected
});
“`
Consuming a Promise
Promises are a far cleaner way to handle asynchronous code in JavaScript than callbacks. They also make it easier to compose asynchronous code.
A promise represents the result of an asynchronous operation. A promise can be in one of three states:
-Pending: The async operation has not yet completed, so the promise is pending.
-Fulfilled: The async operation completed successfully, so the promise is fulfilled.
-Rejected: The async operation failed, so the promise is rejected.
Once a promise is fulfilled or rejected, it is said to be settled and can never change state again.
Consuming a Promise
In order to consume a promise, you need to register a callback with the promise’s then() method. This callback will be invoked when the async operation completes (either successfully or with an error). For example:
Promise States
A promise in JavaScript is an object that represents the eventual value returned from the asynchronous operation. It allows you to write asynchronous code in a more synchronous fashion.
A promise can be in one of three states:
-pending: The async operation has not yet completed. The promise’s then() method has not been called yet.
-fulfilled: The async operation has completed successfully and the promise’s then() method has been called with the resulting value.
-rejected: The async operation has failed and the promise’s then() method has been called with the resulting error.
Promise Chaining
Promise chaining is a pattern in which each successive .then() callback is chained off the previous one. This makes for more readable code, and it’s also more efficient because Promises can be resolved asynchronously. In addition, promise chaining automatically handles errors for you. If an error occurs in any .then() callback, it will be passed down to the next .catch() callback in the chain.
Error Handling
In JavaScript, a promise is an object that represents the pending result of an asynchronous operation. A promise can be in one of three states: pending, fulfilled, or rejected.
Promises are used for error handling in JavaScript because they offer a way to write asynchronous code that is easier to read and debug than traditional callback-based code. When you use promises, you can avoid “callback hell” by chaining together multiple asynchronous operations instead of nesting them.
If an error occurs in an asynchronous operation that is wrapped in a promise, the promise will be rejected and the error will be thrown. You can handle errors in promises by using the .catch() method.
For example, let’s say you have a function that makes an HTTP request to a server:
function makeRequest(url) {
return new Promise(function(resolve, reject) {
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
resolve(this.responseText); // succesfully made request – execute resolve handler function passed into the promise as an argument
} else {
reject(this.statusText); // errors – execute reject handler function passed into the promise as an argument
}
};
xhttp.open(“GET”, url, true); // open connection and make request to url
xhttp.send(); // send request
});
}
Conclusion
From the definition and examples given above, we can conclude that a promise in JavaScript is an object that represents the eventual result of an asynchronous operation. A promise can be in one of three states: pending, fulfilled, or rejected. Once a promise is fulfilled or rejected, it is considered settled (or locked-in), meaning that its state can no longer change.
