promises in javascript

1/*
2	A Promise is a proxy for a value not necessarily known when the promise is created. 
3    It allows you to associate handlers with an asynchronous action's eventual success 
4    value or failure reason.
5*/            
6let promise = new Promise((resolve , reject) => {
7  fetch("https://myAPI")
8    .then((res) => {
9      // successfully got data
10      resolve(res);
11    })
12    .catch((err) => {
13      // an error occured
14      reject(err);
15    });          
16});

1//create a Promise
2var p1 = new Promise(function(resolve, reject) {
3  resolve("Success");
4});
5
6//Execute the body of the promise which call resolve
7//So it execute then, inside then there's a throw
8//that get capture by catch
9p1.then(function(value) {
10  console.log(value); // "Success!"
11  throw "oh, no!";
12}).catch(function(e) {
13  console.log(e); // "oh, no!"
14});
15

1var promise = new Promise(function(resolve, reject) {
2  // do some long running async thing…
3  
4  if (/* everything turned out fine */) {
5    resolve("Stuff worked!");
6  }
7  else {
8    reject(Error("It broke"));
9  }
10});
11
12//usage
13promise.then(
14  function(result) { /* handle a successful result */ },
15  function(error) { /* handle an error */ }
16);

1const myPromise = new Promise((resolve, reject) => {
2  setTimeout(() => {
3    resolve('foo');
4  }, 300);
5});
6
7myPromise
8  .then(handleResolvedA, handleRejectedA)
9  .then(handleResolvedB, handleRejectedB)
10  .then(handleResolvedC, handleRejectedC);
11

1// base case
2const promise = new Promise(executor);
3
4// more complex example
5const coinflip = (bet) => new Promise((resolve, reject) => {
6  const hasWon = Math.random() > 0.5 ? true : false;
7  if (hasWon) {
8    setTimeout(() => {
9      resolve(bet * 2);
10    }, 2000);
11  } else {
12    reject(new Error("You lost...")); // same as -> throw new Error ("You lost ...");
13  }
14});

1myPromise.then(
2  function(value) { /* code if successful */ },
3  function(error) { /* code if some error */ }
4);