Using promises

Promises are another way to use callback functions. Rather than integrating the callback function into the method call (as a parameter), we use it as a parameter of the new then(callback) method. This simplifies the reading of JavaScript code in case it uses callback functions.

For an object to use the then(callback) method, it must be a Promise class object. The Promise class is a class defined in JavaScript language.

The Promise Class

A Promise class object uses a callback function of the form callback(resolve, reject) as a parameter of its constructor.

The resolve and reject parameters are functions, which will be called from the promise’s callback:

• When the resolve() function is called, it triggers the then(callback) method.
• When the reject() function is called, it triggers the catch(callback) method.

The resolve() function must be called, otherwise the then(callback) method cannot be executed. On the other hand, calling the...