JavaScript Promises - Questions

While handling nested async tasks using just callbacks, there's high chance to run into what's known as the 'callback hell'. It's simply extremely indented code that's exceptionally difficult to visually digest, maintain, and debug. Learn more in JavaScript Promises — The limitations of callbacks.

The Promises/A spec was drafted by the CommonJS community to kind-of standardize the interface of a promise object, and some other functionalities surrounding promises. Learn more in JavaScript Promises — The history of promises.

The Promises/A+ spec was meant to build upon the Promises/A spec, improving it where was underspecified. Promises in the ECMAScript 2015 standard were made to conform to this Promises/A+ spec. Learn more in JavaScript Promises — The history of promises.

Promises were introduced in the ECMAScript 2015 version (also known as ECMAScript 6). Learn more in JavaScript Promises — The history of promises.

A promise is an intermediary object meant to placehold the outcome of a given async operation. Learn more in JavaScript Promises — What are promises?

Here are four benefits instead of just three:

1. Promises mitigate the extra levels of indentation otherwise present in callback code.
2. Error handling in promises is much easier as compared to that in callbacks.
3. Promises can be passed around a program, since they're merely objects, with different parts of the program responding to the outcome of the underlying async operation as they want to.
4. Using async/await, which are made to be used with promises, we can make async code look as if it's synchronous code.

Learn more in JavaScript Promises — Benefits of using promises.

The Promise interface is used to work with promises in JavaScript.

Using the Promise() constructor. Learn more in JavaScript Promises — Creating a promise.

The argument to Promise() must be a function. Learn more in JavaScript Promises — Creating a promise.

As per the ECMAScript spec, the argument to Promise() is referred to as the executor function. Learn more in JavaScript Promises — Creating a promise.

The executor function is used to execute an asynchronous operation, and then resolve or reject the underlying promise in the future when the async operation completes (either due to success or failure). Learn more in JavaScript Promises — Creating a promise.

The arguments provided to the executor function are conventionally named resolve and reject, in that very order. Learn more in JavaScript Promises — Creating a promise.

The arguments provided to the executor function, namely resolve and reject, are meant to either resolve or reject the underlying promise, respectively, to ultimately execute all the callbacks set up on the promise (for e.g. by virtue of invocations of then()). Learn more in JavaScript Promises — Creating a promise.

The three states are as follows:

• Pending — the underlying async operation of the promise is still ongoing in the background.
• Fulfilled — the underlying async operation of the promise has successfully completed.
• Rejected — the underlying async operation of the promise has failed.

Learn more in JavaScript Promises — States of a promise.

An unsettled promise is just another way to refer to a pending promise. Learn more in JavaScript Promises — States of a promise.

A settled promise is one that has been either resolved or rejected. Or another way to look at it is that a settled promise is one that is not pending. Learn more in JavaScript Promises — States of a promise.

The value of a promise is the datum with which it either gets resolved or gets rejected (in this case, it's usually an error object). Learn more in JavaScript Promises — The value of a promise.

The then() method is used to set up a callback to be executed when the underlying promise gets resolved (via its first argument) or rejected (via its second argument). Learn more in JavaScript Promises — The then() method.

The arguments provided to then() are conventionally referred to as onFulfilled and onRejected, in that very order. Learn more in JavaScript Promises — The then() method.

True; the Promise() constructor invokes the executor function provided to it immediately, synchronously. Learn more in JavaScript Promises — The executor function.

False; the Promise() constructor requires an argument to be provided to it, and that it must be a function. Learn more in JavaScript Promises — The executor function.

The respective callback is queued up in an internal callback queue maintained by the promise. This queue is emptied and each callback in there executed, as a microtask, when the promise is resolved or rejected (by virtue of the promise's executor calling resolve() or reject(), respectively). Learn more in JavaScript Promises — How then() works under the hood?

The respective callback is executed immediately, as a microtask. Learn more in JavaScript Promises — How then() works under the hood?