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// 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});

1//Promise to load a file...
2//use with openFile(url, fCallback);
3//fCallback(fileContents){ //do something with fileContents}
4const loadFile = url => {
5    return new Promise(function(resolve, reject) {
6        //Open a new XHR
7        var request = new XMLHttpRequest();
8        request.open('GET', url);
9        // When the request loads, check whether it was successful
10        request.onload = function() {
11            if (request.status === 200) {
12                // If successful, resolve the promise
13                resolve(request.response);
14            } else {
15                // Otherwise, reject the promise
16                reject(Error('An error occurred while loading image. error code:' + request.statusText));
17            }
18        };
19        request.send();
20    });
21};
22const openFile = (url, processor) => {
23    loadFile(url).then(function(result) {
24            processor(result);
25        },
26        function(err) {
27            console.log(err);
28        });
29};