Methods that are static and defined within the Promise constructor

Question

Methods that are static and defined within the Promise constructor

While examining the static methods of the Promise constructor, I noticed that there are two methods called resolve and reject:

console.log(Object.getOwnPropertyNames(Promise))
// Array(7) [ "all", "race", "reject", "resolve", "prototype", "length", "name" ]

I started wondering whether these resolve and reject methods are the same ones used as parameters in the executor function, or if they are separate entities altogether:

const myFirstPromise = new Promise((resolve, reject) => {
//   do something asynchronous which eventually calls either:
//
//   resolve(someValue); // fulfilled
//   or
//   reject("failure reason"); // rejected
});

The specification talks about Promise Resolve Functions and %Promise_resolve% intrinsic object known as Promise.resolve ( x ). Can someone clarify if these refer to the same thing?

javascript es6-promise

Answer 1

Answer №1

When you view the contents displayed by console.log(), they are actually referring to the properties .resolve() and .reject() of the global JavaScript Promise object. These can be thought of as static methods within the Promise class.

Essentially, they allow for the creation of new Promise objects that are either resolved or rejected from the start:

const p1 = Promise.resolve(3);
console.log(await p1);
// 3

The parameters resolve and reject that you use in your function call:

const myFirstPromise = new Promise((resolve, reject) => {
   if (rand() < 0.5) {
       resolve(3);
   } else {
       reject(new Error('not today'));
   }
});

are simply variables within the function itself and do not have any direct connection to Promise.resolve() and Promise.reject().

You could alternatively write the code like this:

const f1 = (resolve, reject) => {
   if (rand() < 0.5) {
       resolve(3);
   } else {
       reject(new Error('not today'));
   }
};

const myFirstPromise = new Promise(f1);

In this format, it becomes clearer that resolve and reject are local variables within function f1 and are independent of myFirstPromise or any other instances of Promise.

Answer 2

When you view the contents displayed by console.log(), they are actually referring to the properties .resolve() and .reject() of the global JavaScript Promise object. These can be thought of as static methods within the Promise class.

Essentially, they allow for the creation of new Promise objects that are either resolved or rejected from the start:

const p1 = Promise.resolve(3);
console.log(await p1);
// 3

The parameters resolve and reject that you use in your function call:

const myFirstPromise = new Promise((resolve, reject) => {
   if (rand() < 0.5) {
       resolve(3);
   } else {
       reject(new Error('not today'));
   }
});

are simply variables within the function itself and do not have any direct connection to Promise.resolve() and Promise.reject().

You could alternatively write the code like this:

const f1 = (resolve, reject) => {
   if (rand() < 0.5) {
       resolve(3);
   } else {
       reject(new Error('not today'));
   }
};

const myFirstPromise = new Promise(f1);

In this format, it becomes clearer that resolve and reject are local variables within function f1 and are independent of myFirstPromise or any other instances of Promise.

Answer 3

Answer №2

Utilizing Promise.resolve serves as a quick solution for "taking a value or a promise and enclosing it within another promise". By using

myPromise = Promise.resolve("myVal")

, you can achieve the same outcome in a more concise manner.

myPromise = new Promise((resolve) => resolve("myVal"))

The functionality of Promise.reject mirrors that of Promise.resolve, but instead of resolving, it explicitly rejects the promise.

Answer 4

Utilizing Promise.resolve serves as a quick solution for "taking a value or a promise and enclosing it within another promise". By using

myPromise = Promise.resolve("myVal")

, you can achieve the same outcome in a more concise manner.

myPromise = new Promise((resolve) => resolve("myVal"))

The functionality of Promise.reject mirrors that of Promise.resolve, but instead of resolving, it explicitly rejects the promise.

Methods that are static and defined within the Promise constructor

Answer №1

Answer №2

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