Read a file one character at a time in node.js?

Question

Is there a way to read one symbol at a time in nodejs from file without storing the whole file in memory? I found an answer for lines

I tried something like this but it doesn't help:

const stream = fs.createReadStream("walmart.dump", {
    encoding: 'utf8',
    fd: null,
    bufferSize: 1,
});
stream.on('data', function(sym){
    console.log(sym);
});

Answer 1

Readable stream has a read() method, where you can pass the length, in bytes, of every chunk to be read. For example:

var readable = fs.createReadStream("walmart.dump", {
    encoding: 'utf8',
    fd: null,
});
readable.on('readable', function() {
  var chunk;
  while (null !== (chunk = readable.read(1) /* here */)) {
    console.log(chunk); // chunk is one byte
  }
});

Answer 2

Here's a lower-level way to do it: fs.read(fd, buffer, offset, length, position, callback)

using:

const fs = require('fs');

// open file for reading, returns file descriptor
const fd = fs.openSync('your-file.txt','r');

function readOneCharFromFile(position, cb){

        // only need to store one byte (one character)
        const b = new Buffer(1);

        fs.read(fd, b, 0, 1, position, function(err,bytesRead, buffer){
            console.log('data => ', String(buffer));
            cb(err,buffer);
        });

}

you will have to increment the position, as you read the file, but it will work.

here's a quick example of how to read a whole file, character by character

Just for fun I wrote this complete script to do it, just pass in a different file path, and it should work

const async = require('async');
const fs = require('fs');
const path = require('path');


function read(fd, position, cb) {

    let isByteRead = null;
    let ret = new Buffer(0);

    async.whilst(
        function () {
            return isByteRead !== false;
        },
        function (cb) {
            readOneCharFromFile(fd, position++, function (err, bytesRead, buffer) {

                if(err){
                    return cb(err);
                }

                isByteRead = !!bytesRead;
                if(isByteRead){
                    ret = Buffer.concat([ret,buffer]);
                }

                cb(null);
            });
        },
        function (err) {
            cb(err, ret);
        }
    );

}


function readOneCharFromFile(fd, position, cb) {

    // only need to store one byte (one character)
    const b = new Buffer(1);
    fs.read(fd, b, 0, 1, position, cb);

}


 /// use your own file here
const file = path.resolve(__dirname + '/fixtures/abc.txt');
const fd = fs.openSync(file, 'r');

// start reading at position 0, position will be incremented
read(fd, 0, function (err, data) {
    if (err) {
        console.error(err.stack || err);
    }
    else {
        console.log('data => ', String(data));
    }
    fs.closeSync(fd);
});

As you can see we increment the position integer every time we read the file. Hopefully the OS keeps the file in memory as we go. Using async.whilst() is OK, but I think for a more functional style it's better not to keep the state in the top of the function (ret and isByteRead). I will leave it as an exercise to the reader to implement this without using those stateful variables.