Question

I would to write a simple NodeJS app, that reads a file, analyzes its lines one-by-one and writes the result. Here is the code sample that works

var lines;
var fs = require('fs');
var data="";
fs.readFile('list.csv', 'ascii', function(err,data){
    if(err) {
        console.error("Could not open file: %s", err);
        process.exit(1);
    }
    var data2=data.split(/[\n\r]+/);
    for(var i=0; i<data2.length; i++){
        /*LISTING ALL THE LIST LINE-BY-LINE */
        console.log(i + data2[i]);
    }
});

I’d like to know why I should write my code inside function(err,data){..*my-code*..}? I tried to declare all the variables as global and write

console.log();

at the end of the code – it seems it just dosen’t execute this code line. So, why cant I write my code outside function(err,data){}?

Answer 1

One of the main features of NodeJS is the non-blocking event loop. When functions take a long time, the Node script doesn’t wait for the function to return, which is why you need to pass in your code in a callback.