How to extract the value of a variable outside Nodejs callback function

Question

I am unable to get the value of the response outside the callback code. It returns undefined outside whereas in the callback it is giving proper result.

function doCall(urlString, callback) {
    request.get(
        urlString,
        null,
        null,
        (err, data, result) => {                              
            var statusCode = result.statusCode;
            return callback(data);
        }
    );
}

const apiResponse = doCall(urlString, function(response) {
    console.log('***************************' + response); //Prints correct result
    return JSON.parse(response);
});

console.log('+++++++++++++++++++++++++' + apiResponse); //Prints undefined

Your function doCall does not return anything, so affecting its return "value" to apiResponse sets its value to undefined. — Kevin Pastor
– Kevin Pastor, Commented Jul 23, 2019 at 19:50
but doCall is returning 'return callback(data)'. but is that not correct? — Megha Shah
– Megha Shah, Commented Jul 23, 2019 at 20:02
@Jeff how will using async help me get a value outside of the function. — Megha Shah
– Megha Shah, Commented Jul 23, 2019 at 20:03
You need to read up on how the event loop and async works in node. You need to setup something like var response = await getResponse() from a promise. — Jeff
– Jeff, Commented Jul 23, 2019 at 20:07

jese_moriarty · Accepted Answer · 2019-07-23 20:56:23Z

1

function doCall(urlString) {
    return new Promise((resolve, reject) => {
        request.get(
            urlString,
            null,
            null,
            (err, data, result) => {
                if (error) reject(error);
                var statusCode = result.statusCode;
                resolve(data);
            });
    });
}

async function myBackEndLogic() {
    try {
        const result = await doCall(urlString);
        console.log(result);
       //return JSON.parse(result) if you want

    } catch (error) {
        console.error('ERROR:');
        console.error(error);
    }
}

myBackEndLogic();

Read this for more explanations

edited Jul 23, 2019 at 20:56

answered Jul 23, 2019 at 20:46

jese_moriarty

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4 Comments

Megha Shah Over a year ago

Thank you so much for sharing this. I was able to run the code.

jese_moriarty Over a year ago

Welcome. Read the link i provided. It will help to clear lots of confusions in nodejs.

Jeff Over a year ago

I would also highly recommend this course on Udemy (udemy.com/the-complete-nodejs-developer-course-2). Even if you have a pretty good background in programming it covers a lot of things that are unique/different with nodejs.

Megha Shah Over a year ago

Thank you Jese and Jeff. The links are very useful. I am very grateful. Thank you!

Suman Saini · Accepted Answer · 2019-07-23 20:35:16Z

0

If you want synchronous looking code, wrap everything in an async function:

(async (){
    async function doCall(urlString, callback) {
        return await request.get(urlString, null, null); // or store in a variable and return modified response
    }
    const apiResponse= await doCall(urlString, (response) => {
        console.log('response', response);
        return JSON.parse(response);
    });
    console.log('apiResponse', apiResponse);
})()

answered Jul 23, 2019 at 20:35

Suman Saini

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