Promises are the alternatives of callbacks for delivering the results of asnchronous computations. From the implementation point of view callbacks are easier to implement, promises need some efforts. A promise is used for asynchronous calculations.

The core idea behind promises promise represents the results of an asynchronous operation.

A promise have 3 states:

1: Pending

This is the initial state of the promise.

2. Fulfilled

This represent that the operation is successful.

3: Rejected

This represent that the operation is failed.

Example:

var Promise = require('promise');
ar promise = new Promise(function(resolve, reject){
            request("http://www.google.com", function(error, response, body) {
                console.log("google block...");
                if (error) { reject(error) }
                else resolve(body);
            });
});
promise
.then(function(result){
    console.log(result)
})
.catch(function(err) {
    console.log("This is error block")
    console.log(err);
})
.finally(function() {
    console.log("Final block..");
})

You can get the result of both the function at final:-

 var asyncFunc1 = function() {
        return "test1"
    };
    var asyncFunc2 = function() {
         return "test2"
    }
    Promise.all([
        asyncFunc1(),
        asyncFunc2(),
    ])
    .then(([result1, result2]) => {
        console.log(result1)
        console.log(result2);
    })
    .catch(err => {
        // Receives first rejection among the Promises
        onsole.log("erro block");
        console.log(result1);
        console.log(result2);
    });
    /*Result:
    result1
    result2
    */