Receiving data via stdin and storing as a variable/array

Question

I am trying to receive data via stdin which I have working:

const fs = require('fs');
const readStream = process.stdin;

readStream.pause();
readStream.setEncoding('utf8');

readStream.on('data', (data) => {
  console.log(data);
  });
readStream.resume();

Now what I need to do is store it as a variable so I can do some calculations before I return it via stdout.

Every time I try to so anything with it, like push certain data to an array it repeats the data until the stdin has finished, and I cant access it after it has finished. I cant find any resources online to help me.

I only have experience with JavaScript and would like to keep it as close to what I know as possible. — Tristan
Yes, the requirement is possible using JavaScript available at browser. Are you trying to meet requirement using only node.js? — guest271314
Using node is definitely preferable, but I am open to any suggestions — Tristan

guest271314 guest271314 · Accepted Answer · 2017-08-01T04:01:58

You can use ReadableStream to process asynchronous tasks. Call .getReader() of ReadableStream instance to get an object which has a .read() method which when called returns an object having value and done properties. The controller passed to the constructor can queue tasks to be performed, the call to read() reads the enqueued data and sets the data at value property.

let n = 0;
let letters = "abcdefghijklmnopqrstuvwxyz";
const results = [];

let readableStream = new ReadableStream({
  pull(controller) {
    if (n < letters.length) controller.enqueue(letters[n++])
    else controller.close();
  }
});

let reader = readableStream.getReader();

let processStream = ({value, done}) => {
  if (done) return reader.closed.then(() => results);
  console.log(value);
  // do stuff
  let next = new Promise(resolve => 
    setTimeout(() => {results.push(value); resolve()}
    , Math.floor(Math.random() * 1200))
  );
  return next.then(() => reader.read())
         .then(data => processStream(data));
}

reader.read()
.then(data => processStream(data))
.then(res => console.log(res));

You can also pipe the ReadableStream and .pipeTo() method of the instance to pipe the enqueued data to a WritableStream instance, where tasks can be performed at both read and write portions of the stream.

const letters = "abcdefghijklmnopqrstuvwxyz";

let RSController = class {
  constructor(input) {
    this.input = input;
    this.n = 0;
  }
  async pull(controller) {
    if (this.n < this.input.length) {
      let curr = await this.handleData(this.input[this.n]);
      ++this.n;
      controller.enqueue(curr);
    } else {
      this.n = 0;
      controller.close();
    }
  }
  cancel(err) {
    console.log(err)
  }
  handleData(data) {
    return new Promise(resolve => {
      // do stuff
      setTimeout(() => {
        resolve(data)
      }, Math.floor(Math.random() * 750))
    })
  }
}

let WSController = class {
  constructor(arr = []) {
    this.results = arr;
  }
  write(data) {
    let dataUp = data.toUpperCase();
    console.log(data, dataUp);
    this.results.push([data, dataUp]);
  }
  close() {
    console.log(JSON.stringify(this.results, null, 2));
  }
  abort(e) {
    console.error(e);
  }
}

let rscontroller = new RSController(letters);

let readableStream = new ReadableStream(rscontroller);

let wscontroller = new WSController([]);

let writableStream = new WritableStream(wscontroller);

readableStream
.pipeTo(writableStream);

Receiving data via stdin and storing as a variable/array

1 Answers