nodejs - How to promisify http.request? reject got called two times

I'm trying to wrap http.request into Promise:

 new Promise(function(resolve, reject) {
    var req = http.request({
        host: '127.0.0.1',
        port: 4000,
        method: 'GET',
        path: '/api/v1/service'
    }, function(res) {
        if (res.statusCode < 200 || res.statusCode >= 300) {
            // First reject
            reject(new Error('statusCode=' + res.statusCode));
            return;
        }
        var body = [];
        res.on('data', function(chunk) {
            body.push(chunk);
        });
        res.on('end', function() {
            try {
                body = JSON.parse(Buffer.concat(body).toString());
            } catch(e) {
                reject(e);
                return;
            }
            resolve(body);
        });
    });
    req.on('error', function(err) {
        // Second reject
        reject(err);
    });
    req.write('test');
}).then(function(data) {
    console.log(data);
}).catch(function(err) {
    console.log(err);
});

If I recieve errornous statusCode from remote server it will call First reject and after a bit of time Second reject. How to make properly so it calls only single reject (I think First reject is proper one in this case)? I think I need to close res myself, but there is no close() method on ClientResponse object.

UPD: Second reject triggers very rarely - why?

Your code is almost fine. To restate a little, you want a function that wraps http.request with this form:

function httpRequest(params, postData) {
    return new Promise(function(resolve, reject) {
        var req = http.request(params, function(res) {
            // on bad status, reject
            // on response data, cumulate it
            // on end, parse and resolve
        });
        // on request error, reject
        // if there's post data, write it to the request
        // important: end the request req.end()
    });
}

Notice the addition of params and postData so this can be used as a general purpose request. And notice the last line req.end() -- which must always be called -- was missing from the OP code.

Applying those couple changes to the OP code...

function httpRequest(params, postData) {
    return new Promise(function(resolve, reject) {
        var req = http.request(params, function(res) {
            // reject on bad status
            if (res.statusCode < 200 || res.statusCode >= 300) {
                return reject(new Error('statusCode=' + res.statusCode));
            }
            // cumulate data
            var body = [];
            res.on('data', function(chunk) {
                body.push(chunk);
            });
            // resolve on end
            res.on('end', function() {
                try {
                    body = JSON.parse(Buffer.concat(body).toString());
                } catch(e) {
                    reject(e);
                }
                resolve(body);
            });
        });
        // reject on request error
        req.on('error', function(err) {
            // This is not a "Second reject", just a different sort of failure
            reject(err);
        });
        if (postData) {
            req.write(postData);
        }
        // IMPORTANT
        req.end();
    });
}

This is untested, but it should work fine...

var params = {
    host: '127.0.0.1',
    port: 4000,
    method: 'GET',
    path: '/api/v1/service'
};
// this is a get, so there's no post data

httpRequest(params).then(function(body) {
    console.log(body);
});

And these promises can be chained, too...

httpRequest(params).then(function(body) {
    console.log(body);
    return httpRequest(otherParams);
}).then(function(body) {
    console.log(body);
    // and so on
});

I know this question is old but the answer actually inspired me to write a modern version of a lightweight promisified HTTP client. Here is a new version that:

• Use up to date JavaScript syntax
• Validate input
• Support multiple methods
• Is easy to extend for HTTPS support
• Will let the client decide on how to deal with response codes
• Will also let the client decide on how to deal with non-JSON bodies

Code below:

function httpRequest(method, url, body = null) {
    if (!['get', 'post', 'head'].includes(method)) {
        throw new Error(`Invalid method: ${method}`);
    }

    let urlObject;

    try {
        urlObject = new URL(url);
    } catch (error) {
        throw new Error(`Invalid url ${url}`);
    }

    if (body && method !== 'post') {
        throw new Error(`Invalid use of the body parameter while using the ${method.toUpperCase()} method.`);
    }

    let options = {
        method: method.toUpperCase(),
        hostname: urlObject.hostname,
        port: urlObject.port,
        path: urlObject.pathname
    };

    if (body) {
        options.headers = {'Content-Length':Buffer.byteLength(body)};
    }

    return new Promise((resolve, reject) => {

        const clientRequest = http.request(options, incomingMessage => {

            // Response object.
            let response = {
                statusCode: incomingMessage.statusCode,
                headers: incomingMessage.headers,
                body: []
            };

            // Collect response body data.
            incomingMessage.on('data', chunk => {
                response.body.push(chunk);
            });

            // Resolve on end.
            incomingMessage.on('end', () => {
                if (response.body.length) {

                    response.body = response.body.join();

                    try {
                        response.body = JSON.parse(response.body);
                    } catch (error) {
                        // Silently fail if response is not JSON.
                    }
                }

                resolve(response);
            });
        });
        
        // Reject on request error.
        clientRequest.on('error', error => {
            reject(error);
        });

        // Write request body if present.
        if (body) {
            clientRequest.write(body);
        }

        // Close HTTP connection.
        clientRequest.end();
    });
}

There are other ways as well but here you can find a simple way to make http.request as a promise or async/await type.

Here is a working sample code:

var http = require('http');

function requestAsync(name) {

    return new Promise((resolve, reject) => {
        var post_options = {
            host: 'restcountries.eu',
            port: '80',
            path: `/rest/v2/name/${name}`,
            method: 'GET',
            headers: {
                'Content-Type': 'application/json'
            }
        };
        let post_req = http.request(post_options, (res) => {
            res.setEncoding('utf8');
            res.on('data', (chunk) => {
                resolve(chunk);
            });
            res.on("error", (err) => {
                reject(err);
            });
        });
        post_req.write('test');
        post_req.end();
    });
}

//Calling request function
//:1- as promise
requestAsync("india").then(countryDetails => {
    console.log(countryDetails);
}).catch((err) => {
    console.log(err);  
}); 

//:2- as await
let countryDetails = await requestAsync("india");