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