Essentially, a promise is a returned object you attach callbacks to, instead of passing callbacks into a function. There are three states a Promised can be in: 1. Promises are used for asynchronous handling of events. How to operate callback-based fs.lstat() method with promises in Node.js ? Comprehension is enhanced by seeing the errors actually occur. Promises are used to handle asynchronous http requests. Note that promises are guaranteed to be asynchronous. Since all web APIs are aware of the incumbent settings object, the following will work in all browsers: The same concept applies to promises. Therefore, an action for an already "settled" promise will occur only after the stack has cleared and a clock-tick has passed. Content is available under these licenses. The first promise in the chain is most deeply nested and is the first to pop. // To experiment with error at set-up, uncomment the following 'throw'. It creates a promise that will be fulfilled, using window.setTimeout(), to the promise count (number starting from 1) every 1-3 seconds, at random. A Promise is a special JavaScript object. Having them in JavaScript rather than the DOM is great because they'll be available in non-browser JS contexts such as Node.js (whether they make use of them in their core APIs is another question). That can cause some unexpected behavior in relation to promises. Similarly, .catch() is really just a .then() without a slot for handleFulfilled. Promises replaced callback functions that were used to handle asynchronous operations. To force more errors, change the threshold values. In JavaScript, a promise is an object that represents an asynchronous operation. The effect is much like that of setTimeout(action,10). The result of promise can either be a success or a failure. A Promise is a JavaScript object that links "Producing Code" and "Consuming Code". Note: A promise is said to be settled if it is either fulfilled or rejected, but not pending. Please use, To learn about the way promises work and how you can use them, we advise you to read Using promises first. Promises are used to handle asynchronous operations in JavaScript. Promises can be consumed by registering functions using .then and .catch methods. This is because without tracking the incumbent, we may end up using the wrong environment to send the message. For example, if we are requesting some data from a server, the promise promises us to get that data that we can use in the future. As the Promise.prototype.then() and Promise.prototype.catch() methods return promises, they can be chained. In Wicked Detail. This example shows diverse techniques for using Promise capabilities and diverse situations that can occur. Another simple example using Promise and XMLHttpRequest to load an image is available at the MDN GitHub js-examples repository. The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. The returned nextValue can be another promise object, in which case the promise gets dynamically inserted into the chain. A Promise object is created using the new keyword and its constructor. Multiple callbacks may be added by calling then() several times. edit This code can be run under NodeJS. A realm can be roughly thought of as the global object. Pending 2. If we modify the above example a little bit, we get this: If we change this so that the