Promise.resolve() code example

Example 1: js create a promise

/*
	A Promise is a proxy for a value not necessarily known when the promise is created. 
    It allows you to associate handlers with an asynchronous action's eventual success 
    value or failure reason.
*/            
let promise = new Promise((resolve , reject) => {
  fetch("https://myAPI")
    .then((res) => {
      // successfully got data
      resolve(res);
    })
    .catch((err) => {
      // an error occured
      reject(err);
    });          
});

Example 2: resolve vs return promise js

In simple terms, inside a then handler function:

A) When x is a value (number, string, etc):
	- return x is equivalent to return Promise.resolve(x)
	- throw x is equivalent to return Promise.reject(x)

B) When x is a Promise that is already settled (not 
pending anymore):
	- return x is equivalent to return Promise.resolve(x), 
      if the Promise was already resolved.
	- return x is equivalent to return Promise.reject(x), 
      if the Promise was already rejected.
      
C) When x is a Promise that is pending:
	- return x will return a pending Promise, and it will 
    be evaluated on the subsequent then.
Read more on this topic on the Promise.prototype.then() docs.

Example 3: Promise.resolve

var original = Promise.resolve(33);
original.then( val => console.log('origianl', val);
var cast = Promise.resolve(original);
cast.then(function(value) {
  console.log('value: ' + value);
});
console.log('original === cast ? ' + (original === cast));

// logs, in order:
// original === cast ? true
// value: 33

Example 4: immediate promise resolve

Promise.resolve().then(() => {
    console.log('resolved');
  });
  console.log('end');

// end
// resolve