Tarrence Tarrence - 3 years ago 166
Javascript Question

Removing nested promises

I'm new to promises and writing network code using requests and promises in NodeJS.

I would like to remove these nested promises and chain them instead, but I'm not sure how I'd go about it/whether it is the right way to go.

exports.viewFile = function(req, res) {
var fileId = req.params.id;
boxContentRequest('files/' + fileId + '/content', req.user.box.accessToken)
.then(function(response) {
boxViewerRequest('documents', {url: response.request.href}, 'POST')
.then(function(response) {
boxViewerRequest('sessions', {document_id: response.body.id}, 'POST')
.then(function(response) {
console.log(response);
});
});
});
};


This is the request code:

var baseContentURL = 'https://api.box.com/2.0/';
var baseViewerURL = 'https://view-api.box.com/1/';

function boxContentRequest(url, accessToken) {
return new Promise(function (resolve, reject) {
var options = {
url: baseContentURL + url,
headers: {
Authorization: 'Bearer ' + accessToken,
}
};
request(options, function (err, res) {
if (err) {
return reject(err);
} else if (res.statusCode !== 200) {
err = new Error("Unexpected status code: " + res.statusCode);
err.res = res;
return reject(err);
}
resolve(res);
});
});
}

function boxViewerRequest(url, body, method) {
return new Promise(function (resolve, reject) {
var options = {
method: method,
url: baseViewerURL + url,
headers: {
Authorization: 'Token ' + config.box.viewerApiKey
},
json: body
};
request(options, function (err, res, body) {
if (err) {
return reject(err);
} else if (res.statusCode !== 200 && res.statusCode !== 201 && res.statusCode !== 202) {
err = new Error("Unexpected status code: " + res.statusCode);
err.res = res;
return reject(err);
}
resolve(res, body);
});
});
}


Any insight would be appreciated.

Answer Source

From every then callback, you will need to return the new promise:

exports.viewFile = function(req, res) {
    var fileId = req.params.id;
    boxContentRequest('files/' + fileId + '/content', req.user.box.accessToken)
      .then(function(response) {
          return boxViewerRequest('documents', {url: response.request.href}, 'POST');
      })
      .then(function(response) {
          return boxViewerRequest('sessions', {document_id: response.body.id}, 'POST');
      })
      .then(function(response) {
          console.log(response);
      });
};

The promise that is returned by the .then() call will then resolve with the value from the "inner" promise, so that you easily can chain them.

Generic pattern:

somePromise.then(function(r1) {
    return nextPromise.then(function(r2) {
        return anyValue;
    });
}) // resolves with anyValue

     ||
    \||/
     \/

somePromise.then(function(r1) {
    return nextPromise;
}).then(function(r2) {
    return anyValue;
}) // resolves with anyValue as well
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