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  • Promises in node.js

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    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
        */

     

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