CODEWITHSUNDEEP
node.jsfileasynchronouspromiseasync-await
Maurdekye
Maurdekye

Reputation: 3697

Asynchronous file read reading different number of lines each time, not halting

I built a simple asynchronous implementation of the readlines module built into nodejs, which is simply a wrapper around the event-based module itself. The code is below;

const readline = require('readline');

module.exports = {
  createInterface: args => {
    let self = {
      interface: readline.createInterface(args),
      readLine: () => new Promise((succ, fail) => {
        if (self.interface === null) {
          succ(null);
        } else {
          self.interface.once('line', succ);
        }
      }),
      hasLine: () => self.interface !== null
    };
    self.interface.on('close', () => {
      self.interface = null;
    });
    return self;
  }
}

Ideally, I would use it like so, in code like this;

const readline = require("./async-readline");

let filename = "bar.txt";

let linereader = readline.createInterface({
  input: fs.createReadStream(filename)
});

let lines = 0;
while (linereader.hasLine()) {
  let line = await linereader.readLine();
  lines++;
  console.log(lines);
}

console.log("Finished");

However, i've observed some erratic and unexpected behavior with this async wrapper. For one, it fails to recognize when the file ends, and simply hangs once it reaches the last line, never printing "Finished". And on top of that, when the input file is large, say a couple thousand lines, it's always off by a few lines and doesn't successfully read the full file before halting. in a 2000+ line file it could be off by as many as 20-40 lines. If I throw a print statement into the .on('close' listener, I see that it does trigger; however, the program still doesn't recognize that it should no longer have lines to read.

Upvotes: 0

Views: 562

Answers (1)

Maurdekye
Maurdekye

Reputation: 3697

It seems that in nodejs v11.7, the readline interface was given async iterator functionality and can simply be looped through with a for await ... of loop;

const rl = readline.createInterface({
  input: fs.createReadStream(filename);
});

for await (const line of rl) {
  console.log(line)
}

How to get synchronous readline, or "simulate" it using async, in nodejs?

Upvotes: 2

Related Questions