LineReader

A simple LineReader class, similar to Java's readline() method, written in Typescript (for Node) to read files line-by-line asynchronously and quickly.

Install

# using npm
npm install @dwesley/linereader

Usage

Import

import { LineReader } from "@dwesley/linereader"

Example

import { PathLike } from "node:fs"
import { EOL } from "node:os"

import { LineReader } from "@dwesley/linereader"

async function main() {
  /** could be any valid path to a readable file */
  const PATH: PathLike = "/dev/stdin"

  /** the variable value below can be any valid encoding available in the `BufferEncoding` type */
  const ENCODING: BufferEncoding = "utf8"

  /** `LineReader` instance from the file specified in `PATH` variable  */
  const lineReader = LineReader.create(PATH, ENCODING)

  const output = []
  while (lineReader.hasNextLine()) {
    const line = await lineReader.nextLine()
    output.push(line)
  }
  console.log(output.join(EOL))
}

main()

API

LineReader.create(path: fs.PathLike, encoding: BufferEncoding = "utf8"): LineReaderInstance

The options you can pass are:

Name	Type	Default	Description
path	string | Buffer | URL	none	The path or location of your file (required)
encoding	'ascii' | 'utf8' | 'utf-8' | 'utf16le' | 'ucs2' | 'ucs-2' | 'base64' | 'latin1' | 'binary' | 'hex'	'utf8'	Character encoding to use on read() operation

Instance Methods

The methods of LineReader instance you can access are:

Name	ReturnType	Description
hasNextLine	boolean	A function that returns true if there is another line to read, or false if close() has already been called
nextLine	Promise	An async function that reads the next line from the file and returns it.
close	void	A function that closes the LineReader and stops reading lines from the file.

LineReaderInstance.nextLine<Output>(fn?: (value: string) => Output): Promise<Output | string | undefined>

Asynchronously read next single line of current file stream.

Example:

const lineReader = LineReader.create("./file.txt", "utf-8")
const output = []
while (lineReader.hasNextLine()) {
  const line = await lineReader.nextLine()
  output.push(line)
}

console.log(output.join(" | "))

./file.txt

1111
2222
3333
4444
5555

7777

Output:

1111 | 2222 | 3333 | 4444 | 5555 | | 7777

Example with helpers:

/** Helper function that should format the current line */
const helper = (line: string = ""): number[] => {
  return line.split(",", 1e5).map((value) => Number.parseInt(value, 10))
}

const output = []
while (lineReader.hasNextLine()) {
  /**
   * Extended nextLine method for lineReader instance
   * This nextLine method uses `helper`function as proxy for return the line content
   * Its return type is inherited from helper's return type
   */
  const numbers = await lineReader.nextLine(helper)
  output.push(numbers.join("-"))
}
console.log(output.join(EOL))

./using-helper.txt

1,2,3
4
5,,7

Output:

1-2-3
4
5-NaN-7

LineReaderInstance.close(): void

Manually closes the LineReader by calling the close method. This method will be called automatically on the last nextLine operation.

Example:

const lineReader = LineReader.create("./file.txt")

for (let i = 0; i < 2; i++) {
  const line = await lineReader.nextLine()
  else console.log(line)
}

if (lineReader.hasNextLine())
  lineReader.close()

console.log(await lineReader.nextLine()) // undefined

./file.txt

1
2
3
4

Output:

1
2
undefined

Contribute

This is open for you to join if you can add value or benefit from this!

License

Feel free to use this library under the conditions of the MIT license.