How to call an api from another api in expressjs?

Create a common middleware which need to executed for both the routes.

Below is the code snippet for the same:

app.get('/test', test);
app.get('/check', check, test);

check and test are the middlewares which is used in common.

Simple solution is to define a method which can be called using both request routes.

app.get('/test', (req, res) => {
   console.log("this is test");
    callMeMayBe();
});

callMeMayBe()
{
    //Your code here
}

To "call an API from another API", a quick and easy way is sending HTTP request inside Express server, browser would never know an internal HTTP invocation happens, not mention page-redirect. The benefit of this design includes:

1. There's no need to change the current API design.
2. The API invocation can be made exact like sent from browser.

Here is an example:

var http = require('http');

router.get('/test', function(req, res) {
  res.end('data_from_test');
});

router.get('/check', function(req, res) {
  var request = http.request({
    host: 'localhost',
    port: 3000,
    path: '/test',
    method: 'GET',
    headers: {
      // headers such as "Cookie" can be extracted from req object and sent to /test
    }
  }, function(response) {
    var data = '';
    response.setEncoding('utf8');
    response.on('data', (chunk) => {
      data += chunk;
    });
    response.on('end', () => {
      res.end('check result: ' + data);
    });
  });
  request.end();
});

The result of GET /check would be:

check result: data_from_test