Node.js, using async/await with readline.on function

Question

I want to read a file which has an kye and value.

I am using 'readline' to read line by line and store it to map object.

But it is not working and only shows that 'undefined'.

Do you guys have any ideas on how to fix it?

Thanks a lot in advance

#!/usr/bin/env node
const fs = require('fs');
const readline = require('readline');
const hg = require('./js/funtions.js');
if (require.main === module) {
  const args = process.argv
  var propertiesPath;
  if(args.length >= 3){
    propertiesPath = args[2];
  }else {
    console.log("No properties path");
    process.exit(1);
  }
  if (propertiesPath.includes("-p")) {
    propertiesPath = propertiesPath.replace("-p","");
  }

  const file = readline.createInterface({
    input: fs.createReadStream(propertiesPath),
    output: process.stdout,
    terminal: false
  });
  var map = new Map();
  var tokens,key,value;
  file.on('line', (line) => {
    tokens = line.split("=")
    key   = tokens[0];
    value = tokens[1];
    map.set(key,value);
  });
 
  var jsonPath = map.get("jsonPath");
  console.log(jsonPath);
}

t.niese · Accepted Answer · 2020-12-28 11:12:37Z

1

using async/await with readline.on function

You don't use await/async in you code.

Besides that file.on('line', …) registers a callback to be called for each line the stream encounters and this happens asynchronously. Due to these two lines of code are executed before any line in the file was found by readline:

var jsonPath = map.get("jsonPath");
console.log(jsonPath);

If you want to execute those two lines of code after all lines have been read by the stream you need to do that at the close event:

file.on('close', () => {
  var jsonPath = map.get("jsonPath");
  console.log(jsonPath);
});

answered Dec 28, 2020 at 11:12

t.niese

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2 Comments

Hani Lee Over a year ago

But I still need to use 'jsonPath' to do other work. outside of close function. jsonPath still shows undefined.

t.niese Over a year ago

@HaniLee that's the same problem your console.log(jsonPath); outside of the close callback is called before the lines are read. That's how asynchronous code works. You should do some research about how asynchronous code works, and the possibilities that exist how to work with asynchronous code, e.g. using Promises in combination with await and async.

Hani Lee · Accepted Answer · 2020-12-28 12:19:28Z

After doing research. I decided not to use async/await function. So I used a simple logic to read file and parse to map object. Here is my code.

#!/usr/bin/env node
const fs = require('fs');
const readline = require('readline');
const hg = require('./js/funtions.js');

if (require.main === module) {
  const args = process.argv
  var propertiesPath;
  if(args.length >= 3){
    propertiesPath = args[2];
  }else {
    console.log("No properties path");
    process.exit(1);
  }

  if (propertiesPath.includes("-p")) {
    propertiesPath = propertiesPath.replace("-p","");
  }

  var lines = require('fs').readFileSync(propertiesPath, 'utf-8')
    .split('\n')
    .filter(Boolean);

  var map = new Map();
  var tokens,key,value;
  for (var i = 0; i < lines.length; i++) {
    tokens = lines[i].split("=")
    key   = tokens[0];
    value = tokens[1];
    map.set(key,value);
  }
  var jsonPath = map.get("jsonPath");
  console.log(jsonPath);
}

Thanks a lot.

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