read-last-lines/index.js

const fsp = require('fs-promise');

const newLineCharacters = ["\n", "\r"]

const readPreviousChar = function(stat, file, currentCharacterCount) {
	return fsp.read(file, new Buffer(1), 0, 1, stat.size - 1 - currentCharacterCount)
		.then((bytesReadAndBuffer) => {
			return String.fromCharCode(bytesReadAndBuffer[1][0]);
		})
}

module.exports = {

	/**
	 * Read in the last `n` lines of a file
	 * @param	{string}	 file				 direct or relative path to file.
	 * @param	{int}	 		maxLineCount max number of lines to read in.
	 * @return {promise}							 new Promise, resolved with lines or rejected with error.
	 */

	read: function(input_file_path, maxLineCount) {
		return new Promise((resolve, reject) => {
			let self = {
				stat: null,
				file: null
			}

			fsp.exists(input_file_path)
			.then(function(exists) {
				if (!exists) {
					throw "file does not exist";
				}

			}).then(function() {
				var promises = [];

				// Load file Stats.
				promises.push(
					fsp.stat(input_file_path)
						.then(stat => self.stat = stat));

				// Open file for reading.
				promises.push(
					fsp.open(input_file_path, 'r')
						.then(file => self.file = file));

				return promises;
			}).then(function(promises) {
				Promise.all(promises)
					.then(() => {
						var chars = 0;
						var lineCount = 0;
						var lines = '';

						var do_while_loop = function() {
							if (lines.length > self.stat.size) {
								lines = lines.substring(lines.length - self.stat.size);
							}

							if (lines.length >= self.stat.size || lineCount >= maxLineCount) {
								if (newLineCharacters.includes(lines.substring(0, 1))) {
									lines = lines.substring(1);
								}
								return resolve(lines);
							}

							readPreviousChar(self.stat, self.file, chars)
								.then((nextCharacter) => {
									lines = nextCharacter + lines;
									if (newLineCharacters.includes(nextCharacter) && lines.length > 1) {
										lineCount++;
									}
									chars++;
								})
								.then(do_while_loop);
						}
						do_while_loop();

					});
			}).catch(reject);
		})
	}
}