Error with using Promise function in javascript

Question

I am trying to use a Promise function. The promise function is nested inside the dataService function. And the dataService function has a getData method inside, it defines a promise and then checks , if the number inside generated randomly is greater than 5 then the promise variable executes resolve else its a reject (inbuilt promise parameters). The then property logs the answer. However i am getting errors like - Cannot read property 'then' of undefined and Cannot read property 'resolve' of undefined at Promise.html:19. Please help.

 <html>

 <head>
<title>Promise</title>
</head>

<body>
<script>
   function dataService() {
        var obj = {};
        console.log('inside function');

        obj.getData = function () {
            var promise = new Promise(function(resolve,reject){

                var number = Math.random() * 10;

                if (number > 5) {
                   ( promise.resolve("Resolved promise " + number));

                }
                else
              ( promise.reject("Promise Rejected " + number));

        })};

            return obj;

        }


    var data = dataService();
    console.log(data);

   data.getData()
        .then(function(val) {
            console.log("It has been a success"),
            console.log(val)
         } )
        .catch(function (err) {
                console.log("It has been failed"),
                console.log(err)
        });
  </script>

 </body>

 </html>

mk010101 · Accepted Answer · 2018-03-28 08:29:15Z

You should return the Promise when the function is called:

function dataService() {
        var obj = {};
        console.log('inside function');

        obj.getData = function () {
            return new Promise(function(resolve,reject){

                var number = Math.random() * 10;

                if (number > 5) {
                   ( resolve("Resolved promise " + number));

                }
                else
              ( reject("Promise Rejected " + number));

        })};

            return obj;

        }


    var data = dataService();
    console.log(data);

   data.getData()
        .then(function(val) {
            console.log("It has been a success"),
            console.log(val)
         } )
        .catch(function (err) {
                console.log("It has been failed"),
                console.log(err)
        });

Daniel Hilgarth · Accepted Answer · 2018-03-28 08:24:27Z

0

Don't use promise.resolve and promise.reject. Just use resolve and reject.

Also, you need to actually return the promise from your getData function.

Try educating yourself with a tutorial, e.g. this one: https://developers.google.com/web/fundamentals/primers/promises

edited Mar 28, 2018 at 8:24

answered Mar 28, 2018 at 8:22

Daniel Hilgarth

175k41 gold badges346 silver badges450 bronze badges

1 Comment

java_jazz Over a year ago

still getting errors --> Cannot read property 'then' of undefined. And --> Uncaught (in promise) Promise Rejected 2.0840987564690128

KarelG · Accepted Answer · 2018-03-28 08:27:18Z

0

You are returning with an object instead of a promise object

obj.getData = function () {
    var promise = new Promise(function(resolve,reject){
    // omitted
    return obj; // <---- culprit.
}

just return with promise instead

obj.getData = function () {
    var promise = new Promise(function(resolve,reject){
    // omitted
    return promise; // <---- changed
}

so that when doing data.getData(), you will receive a Promise object. This object has .then and .catch functions.

answered Mar 28, 2018 at 8:27

KarelG

5,2544 gold badges35 silver badges53 bronze badges

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