如何等到 request.get 完成然后在 node.js 中执行下一个块

Question

I am new to NodeJS and I am working on a request.get problem. My goal is simply have a function that request the web, and when request finished, the function returns the result, otherwise it returns an error message.

Here's the function that I used for request:

var artistNameIdMap = {};
var getPopularArtists = async () => {
    //https://nodejs.org/api/http.html#http_http_request_options_callback
    var options = {
        url: CONSTANTS.API_ENDPOINTS.playlist_endpoint + subpath,
        headers: { 'Authorization': 'Bearer ' + access_token,
                    'Accept': 'application/json',
                    'Content-Type': 'application/json'},
        json: true
    }
    
    request.get(options, function(error, response, body) {
        if (response.statusCode === 200){
            console.log("inside");
            artistNameIdMap = getArtistNameIdMap(body, artistNameIdMap);
        } else {
            res.send("get popular error");
            return {};
        }
    })

    console.log("outside");
    return artistNameIdMap;


module.exports = {
    GetPopularArtists: getPopularArtists
}

And this function is included in a getPopular.js file. I would like to call the function in another file playlist.js .

In playlist.js , I wrote

const getPopular = require('./controllers/getPopular');
router.get("/BPM/:BPM", (req, res) =>{
    const artistNameIdMap = getPopular.GetPopularArtists();
    console.log(artistNameIdMap);
    let BPM = req.params.BPM;
    res.send(BPM);
})

However the result I got is

outside
Promise { {} }
inside

It seems like the return was before the request gives back the information. I wonder what should I write to make sure that I can obtain the correct artistNameIdMap at playlist.js .

Answer 1

Since you want to use Promises, use it like this

const getPopularArtists = () => new Promise((resolve, reject) {
    const options = {
        url: CONSTANTS.API_ENDPOINTS.playlist_endpoint + subpath,
        headers: {
            'Authorization': 'Bearer ' + access_token,
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        json: true
    }

    request.get(options, (error, response, body) => {
        if (error) {
            reject(error);
        } else if (response.statusCode === 200) {
            console.log("inside");
            resolve(getArtistNameIdMap(body, artistNameIdMap));
        } else {
            reject("get popular error");
        }
    });
});

module.exports = {
    GetPopularArtists: getPopularArtists
}

And use it like

const getPopular = require('./controllers/getPopular');
router.get("/BPM/:BPM", async (req, res) =>{
    try {
        const artistNameIdMap = await getPopular.GetPopularArtists();
        console.log(artistNameIdMap);
        let BPM = req.params.BPM;
        res.send(BPM);
    } catch(err) {
        res.send(err);
    }
})

---

Alternatively, without promises, you'll need to use a callback

Using callbacks:

const getPopularArtists = (callback) => {
    const options = {
        url: CONSTANTS.API_ENDPOINTS.playlist_endpoint + subpath,
        headers: { 'Authorization': 'Bearer ' + access_token,
                    'Accept': 'application/json',
                    'Content-Type': 'application/json'},
        json: true
    }
    
    request.get(options, function(error, response, body) {
        if (error) {
            callback(error);
        } else if (response.statusCode === 200){
            console.log("inside");
            callback(null, getArtistNameIdMap(body, artistNameIdMap));
        } else {
            callback("get popular error");
        }
    })
};

module.exports = {
    GetPopularArtists: getPopularArtists
}

And use it like:

const getPopular = require('./controllers/getPopular');
router.get("/BPM/:BPM", (req, res) =>{
    getPopular.GetPopularArtists((err, artistNameIdMap) => {
        if (err) {
            // handle error here
        } else {
            console.log(artistNameIdMap);
            let BPM = req.params.BPM;
            res.send(BPM);
        }
    });
});

Answer 2

Though you've already accepted an answer, there are a couple of additional things I can add. First, the request() library has been deprecated and it is not recommended for new code. Second, there is a list of recommended alternatives here . Third, all these alternatives support promises natively as that is the preferred way to program asynchronous code in modern nodejs programming.

My favorite alternative is got() because I find it's interface simple and clean to use and it has the features I need. Here's how much simpler your code would be using got() :

const got = require('got');
let artistNameIdMap = {};

async function getPopularArtists() {
    const options = {
        headers: { 'Authorization': 'Bearer ' + access_token,
                    'Accept': 'application/json',
                    'Content-Type': 'application/json'},
    };
    const url = CONSTANTS.API_ENDPOINTS.playlist_endpoint + subpath;
    let results = await got(url, options).json();
    // update local cache object
    artistNameIdMap = getArtistNameIdMap(results, artistNameIdMap);
    return artistNameIdMap;
}

module.exports = {
    GetPopularArtists: getPopularArtists
}

Note: The caller should supply error handling based on the returned promise.

GetPopularArtists().then(results => {
    console.log(results);
}).catch(err => {
    console.log(err);
});