From Chapter 1, Promises.js, we are already aware that a promise is based on three states. Let's brush up our memory on these states, in accordance with promises paradigm.

Promise has three states:

A promise exists within these three states.

The beginning of a promise is from an unfulfilled state. This is due to the fact that a promise is a proxy for an unknown value.

When the promise is filled with the value it's waiting for, it's in the fulfilled state. The promise will be labeled as failed if it returns an exception.

A promise may move from an unfulfilled to a fulfilled or failed state. Observers (or the objects/events waiting) are notified when the promise is either rejected or fulfilled. Once the promise is rejected or resolved, its output (value or state) cannot be modified.

The following code snippet will help you understand more easily than theory:

// Promise to be filled with future value
var futureValue = new Promise();

// .then() will return a new promise
var anotherFutureValue = futureValue.then();

// Promise state handlers (must be a function ).
// The returned value of the fulfilled / failed handler will be the value of the promise.
futureValue.then({

    // Called if/when the promise is fulfilled
    fulfilledHandler: function() {},

    // Called if/when the promise fails
    errorHandler: function() {},

    // Called for progress events (not all implementations of promises have this)
    progressHandler: function() {}
});