Learning a new framework like Angular2 brings up a large number of technical items that you end up using, but not necessarily understanding right away. For example, a simple DataService had me implement the following code:

getModels(): Promise<Model[]> {
    return Promise.resolve(MODELS);
}

Even though this is a mock object, it is good practice to assume that data operations may take some time - enough where async operations come into play.

Wanting to understand a bit more, I did some quick research and a demo on promises. Here’s what I found out: much of this from MDN

• Promises are a newer language feature added to JavaScript in 2013.
• Promises are used for asynchronous operations.
• The Executor is a function that receives function references resolve and reject
• The Executor is called immediately. At some point in this function, either the resolve or reject functions are be called.
• There are three states to a promise: Pending, Fulfilled, Rejected
• When resolve is called, the then function is called on the promise. The promise is considered fulfilled
• when reject is called, the catch function is called on the promise, and is considered Rejected
• promises can be chained. This allows for a degree of synchronization betweens asynchronous operations.
• note that then always returns a promise, whether one is explicitly created or not. The implicit promise essentially resolves immediately

Basic promise Format

var promise = new Promise(
    function(resolve, reject) {
        //some sort of long running / async operation
        if (true) {
            resolve("success");
        }
        else {
            reject("failure");
        }
    }
);
promise.then( // called when resolve() executed
    function(result) {
        console.log("Success: " + result);
    })
.catch( //called when reject() executed
    function(reason) {
        console.log("Failure: " + reason );
    });

Here is an example JS Fiddle I wrote that demonstrates promises and promise chaining:

jsfiddle.net/brnvndr/Lep6dghu/