gen-readlines

A generator based line reader. This node package will return the lines of a file as a generator when given file descriptor and the size of the file.

Why?

I created this project primarily for better flow control of reading lines in a file. Instead of using callbacks for reading lines within a file, this will use a generator which has some unique benefits.

• Because it is a generator, the lines of the file are not stored in a large array
• Code is synchronous, so you don't have to worry about the asynchronous effects of waiting for a callback
• Performant, our micro-benchmark demonstrates it can compete with callback-based line readers and in fact appears to be faster.

Also, there are no external depencies and the library was built using TypeScript.

Install

npm install gen-readlines

Usage

import { fromFile } from 'gen-readlines';

async function readFile() {
  for (let line of fromFile('./file.txt')) {
    console.log(line.toString());
  }
}

If you already have the file open and you know the filesize you can use gen-readlines and it will create a generator which will iterate through all the lines in that file.

import fs from 'fs';
import util from 'util';
import readlines from 'gen-readlines';

const open = util.promisify(fs.open);
const fstat = util.promisify(fs.fstat);

async function readFile() {
  const fd = await open('./file.txt');
  const stat = await fstat(fd);
  const fileSize = stat.size;

  for (let line of readlines(fd, fileSize)) {
    console.log(line.toString());
  }

  console.log('continue code execution, no callbacks!');

  fs.closeSync(fd);
}

readlines returns a generator object and calling next will get the next line as a buffer object:

var file = readlines(fd, stats.size);
var line = file.next();
console.log(line);
// { value: <Buffer 42 65 73 70 ... >, done: false }

Convert the buffer to a string:

line.toString();
// This is the first line of the file

Maximum Line Length

You can limit the maximum line length. When the specified length is reached while reading a line, the buffer will be returned as a new line just like when a line break was encountered:

// If original lines are longer than 255 characters, an artificial line break
// will be enforced after each 255 characters reached on a single line. More
// then original lines will be returned by the generator then.
var file = readlines(fd, stats.size, { maxLineLength: 255 });
for (let line of readlines(fd, fileSize)) {
  console.log(line.toString());
}

You can change the maximum line length for each generated line. If you do not specify the maximum length, when you read the next line, the original maximum line length passed to readlines will be used:

// Lines will not be longer than 255 characters by default.
var file = readlines(fd, stats.size, { maxLineLength: 255 });
var line = file.next(); // 255 characters maximum
line = file.next(127);  // 127 characters maximum
line = file.next();     // 255 characters maximum again

Note: The very first generation (call to the next method) cannot accept an alternative maximum line length. It will always use the default value passed to readlines. First the following calls to next allow to specify alternative values. This is caused by the nature of JavaScript generators, which obtain the value from yield first when when resuming the generation.

Compatibility

• node 6 or newer, to get access to generators and new methods of the Buffer object.

Documentation

readlines (fd, filesize, { bufferSize=64*1024, position=0, maxLineLength=Infinity })

• fd {Number} The file descriptor
• filesize {Number} The size of the file in bytes
• bufferSize {Number} The size of the buffer in bytes, default: 64*1024
• position {Number} The position where to start reading the file in bytes, default: 0
• maxLineLength {Number} The length to stop reading at if no line break has been reached, default: Infinity

fromFile (filename, { bufferSize=64*1024, maxLineLength=Infinity })

• filename {string} Name of input file
• bufferSize {Number} The size of the buffer in bytes, default: 64*1024
• maxLineLength {Number} The length to stop reading at if no line break has been reached, default: Infinity

Testing

We are using mocha for unit testing

npm test

Performance

./perf contains a micro-benchmark, ran each benchmark five times and then averaged the results:

Package	Runtime (nanoseconds)
readline	17769036.4
gen-readlines	24480520.4
linebyline	26549054.2
byline	41573681.0
line-reader	58315530.0