nodejs express read file and return response asynchronously

Question

I want to do something like this

 router.get('/getforecast/:stationShortCode', function(req,res){
 var stationShortCode = req.params.stationShortCode;

 var fs = require('fs');
 fs.readFile("data/wx.hourly.txt", "utf8", function(err, data){
    if(err) throw err;
     //do operation on data that generates say resultArray;
     return resultArray;
  });

 res.send(resultArray);

});

obviously, res.send is called synchronously, whereas the file reads async. Is there a clever solution to this? Am I forced to actually pass in res?

Answer 1

fs.readFile("data/wx.hourly.txt", "utf8", function(err, data){
    if(err) throw err;

    var resultArray = //do operation on data that generates say resultArray;

    res.send(resultArray);
});

Your file is read. The callback you passed to the readFile function is called. Then you can generate the data, and then you can send the response. Look into how asynchronous callbacks work, because here your code is not executed from top to bottom like you think it is. And because anonymous functions "remember" all the variables from the scope around them, you can use res inside that callback.