You can use the built-in http module in node, or use a third-party package such as request.
HTTP
An example of using the built in http module e.g:
// The 'https' module can also be used
const http = require('http');
// Example route
app.get('/some/api/endpoint',  (req, res) => {
    http.get('http://someapi.com/api/endpoint', (resp) => {
        let data = '';
        // Concatinate each chunk of data
        resp.on('data', (chunk) => {
          data += chunk;
        });
        // Once the response has finished, do something with the result
        resp.on('end', () => {
          res.json(JSON.parse(data));
        });
        // If an error occured, return the error to the user
      }).on("error", (err) => {
        res.json("Error: " + err.message);
      });
});
Request
Alternatively, a third party package such as request can be used. 
First install request:
npm install -s request
And then change your route to something like this:
const request = require('request');
// Example route
app.get('/some/api/endpoint',  (req, res) => {
    request('http://someapi.com/api/endpoint',  (error, response, body) => {
        if(error) {
            // If there is an error, tell the user 
            res.send('An erorr occured')
        }
        // Otherwise do something with the API data and send a response
        else {
            res.send(body)
        }
    });
});