How to return the value from inner function in Nodejs

Question

I have Nodejs code using request library to get the authentication token from a url.

var request = require("request")

const wellish_dev_url = "https://dev.wellish.com"

function get_auth_token(){
    const  api_end_point = wellish_dev_url + "/api/v1/auth"

    var options = {
        url: api_end_point,
        headers: {"Content-Type": "application/json"},
        auth: {
            'user': 'admin',
            'pass': 'password'}
    }

    var r = request.get(options, function(error, response, body){
        if(!error && response.statusCode==200){
            var token = JSON.parse(body)
            var auth_token = token["data"][0]["accessToken"]
            // console.log(auth_token)
            return auth_token
        }
        else{
            console.log("Code : " + response.statusCode)
            console.log("error : " + error)
            console.log('body : ' + body)
        }
    })
}

// get_auth_token()
var auth_token_new = get_auth_token()
console.log(auth_token_new)

I want to return the auth_token to be used as an argument in another function. However, it shows undefined.

I look up online to use cb, however, I have no idea how it works. I really want to return the value without using cb. Any helps?

cb simply means callback - and it works like any other callback would — Derek Pollard
– Derek Pollard, Commented Apr 30, 2019 at 21:11
Possible duplicate of Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference — zero298
– zero298, Commented Apr 30, 2019 at 21:12
I suggest to return a Promise from your function, and resolve it in the inner callback, I can provide an answer with an example if that can help — jo_va
– jo_va, Commented Apr 30, 2019 at 21:16
@jo_va Or you could refer to the proposed duplicate since it pretty much covers every possible topic with respect to asynchronous code in JavaScript. — zero298
– zero298, Commented Apr 30, 2019 at 21:17

jo_va · Accepted Answer · 2019-04-30 22:09:00Z

2

Your callback is only executed once the response comes back from the server. By that time, your function will already have been executed.

Since your code is asynchronous, you need a means to return the response only when you get it back from the server.

You can use a Promise for that task.

Here is an example on how to solve it in your particular case. Use the Promise constructor to return a Promise which resolves/rejects when your inner callback gets called.

Then you simply have to chain a .then() to your function call to get the token:

...

function get_auth_token() {
    const  api_end_point = ...
    const options = ...

    return new Promise((resolve, reject) => {
        var r = request.get(options, (error, response, body) => {
            if (!error && response.statusCode == 200){
                var token = JSON.parse(body)
                var auth_token = token["data"][0]["accessToken"]
                resolve(auth_token)
            } else {
                console.log("Code : " + response.statusCode)
                console.log("error : " + error)
                console.log('body : ' + body)
                reject(error)
            }
        })
    });
}

get_auth_token()
    .then(token => console.log(token))
    .catch(error => console.error(error));

You can also use await to get the value from the Promise like this, just make sure you are using await in a function marked async:

async function parent_function() {
    function get_auth_token() { ... }

    try {
        const token = await get_auth_token();
        console.log(token);
    } catch (error) {
        console.log(error);
    }
}

edited Apr 30, 2019 at 22:09

answered Apr 30, 2019 at 21:24

jo_va

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12 Comments

Akira Over a year ago

I am very new to Nodejs, and I have no idea we have to use Promise and callback, I just want to return the value simply like how python does. Is there any way to return without using Promise and callback

jo_va Over a year ago

@Akira, your callback is executed later in time, once a response comes back from the server. So the code is inherently asynchronous. Your function will be done executing before the response from the server will come, so you need a means to execute your code only once this response comes back, you can use a Promise for that.

the0ffh Over a year ago

While using both resolve and reject it would be nice to provide a line responsible for handling the second one as well, e.g: get_auth_token().then(console.log).catch(console.log) or get_auth_token().then(result => { console.log(result) }).catch(error => { console.log(error) }) they both result's are the same in this case. To avoid confusion stick to the second example (first is for OP's further research ;)).

the0ffh Over a year ago

@Akira, take a look at this question - the answer describes how can you assign results of promise / async/await using example provided by @jo_va

jo_va Over a year ago

@Akira, I suggest you read some material on Promise. This was already explained, using .then and async/await, see my updated answer.

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