Save array made with readline module

Question

I have the next piece of code. It's a function that has to return an array based on a txt file. The problem is when I print piece by piece the array, it prints well. But when I print the array out of the Interface, the array is empty.

const fs = require('fs');
const readline = require('readline');

function read_file(filename) {
   const filePath = './uploads/' + filename;
   var data = []

   data = readline.createInterface({
      input: fs.createReadStream(filePath),
      terminal: false
   }).on('line', function (
      data.push(line);
      console.log(data); // Here the array is filling well
   });

   console.log(data); // Buy here is empty again
}

Answer 1

It's because of an asynchronous architecture of Node.js, your console.log executes before read file task.

If you wanna expect a true result, you must make your function to return promise, and also notice that when your event finish then resolve your data.

Something like this might help you:

const fs = require('fs');
const readline = require('readline');

async function read_file(filename) {
  const filePath = './uploads/' + filename;
  var readData = [];

  let data = await new Promise((resolve, reject) => {
    try {
      readline.createInterface({
        input: fs.createReadStream(filePath),
        terminal: false
      })
        .on('line', function (line) {
          readData.push(line);
        })
        .on('close', function() {
          resolve(readData);
        });
      }
      catch(e) {
       reject(e);
      }
   });

 console.log(data);
}