How to make response data available outside the function - node.js

Question

Data received from the server is written into variable cols. But it is empty in the output. I'm not sure if cols has now a proper value and is available to work with.
app.post('/get_columns', function (req, res, next) {
var cols = '';
var httpreq = http.request(options, function (response) {
response.on('data', function (chunk) {
cols += chunk;
console.log("body: " + cols);
});
});
httpreq.write(options_data);
httpreq.end();
console.log('cols: '+cols);
});
What output shows:
cols:
body: {"ResultSet Output":[{"NAME": "PART_COL1"},{"NAME": "PART_COL2"}],"StatusCode": 200,"StatusDescription": "Execution Successful"}

Answer 1

HTTP Calls are asynchronous call in Node.js and the response comes as multiple chunks through Events. When you are dealing
with HTTP request in Node.js, you have to deal with "data" & "end"
events on the response object given by HTTP.
Below examples shows how you can deal with the response in HTTP:
app.post('/get_columns', function (req, res, next) {
var cols = '';
var httpreq = http.request(options, function (response) {
// This event is triggered multiple times whenever a chunk is available.
response.on('data', function (chunk) {
cols += chunk;
});
// This event is triggered once when all the chunks are completed.
response.on('end', function (chunk) {
console.log("body: " + cols);
});
});
httpreq.write(options_data);
httpreq.end();
});
Another organized way to handle
var fetchCols = function (options, options_data) {
return new Promise((resolve, reject) => {
var cols = '';
var httpreq = http.request(options, function (response) {
// This event is triggered multiple times whenever a chunk is available.
response.on('data', (chunk) => {
cols += chunk;
});
// This event is triggered once when all the chunks are completed.
response.on('end', () => {
resolve(cols);
});
});
// This event is triggered when there is any error.
httpreq.on('error', (e) => {
reject(e);
});
httpreq.write(options_data);
httpreq.end();
});
};
app.post('/get_columns', function (req, res, next) {
fetchCols(options, options_data)
.then((cols) => {
console.log('body: ', cols);
})
.catch((err) => {
console.error('ERR: ', cols);
});
});