CODEWITHSUNDEEP
node.jscsv
NightMICU
NightMICU

Reputation: 9230

Read the last line of a CSV file and extract one value

New to Node.js and trying to pull a value from the very last line of a CSV file. Here is the CSV:

Unit ID,Date,Time,Audio File
Log File Created,3/6/2013,11:18:25 AM,file:\\\C:\Users\Ben\Documents\1_03-06-2013_1114-50.mp3
1,3/6/2013,11:20:24 AM,file:\\\C:\AlertLog\1_03-06-2013_1120-24.mp3
1,3/6/2013,11:20:39 AM,file:\\\C:\AlertLog\1_03-06-2013_1120-24.mp3

The part I am trying to grab is file:\\\C:\AlertLog\1_03-06-2013_1120-24.mp3 - preferably getting rid of the file:\\\ part.

Sorry that I do not have any code to show, just have a few hours of experience with Node.js and cannot seem to find any docs on how to accomplish something like this. Any help would be appreciated. Thanks!

Upvotes: 5

Views: 13432

Answers (2)

Tola
Tola

Reputation: 631

I did it by reading the file backwards until the last line was read:

Update 2022 (use modern javascript and read buffer only once)

import { open } from 'node:fs/promises';
import path from 'node:path';
import { fileURLToPath } from 'url';

const __dirname = path.dirname(fileURLToPath(import.meta.url));
const fileName = path.join(__dirname, 'test.txt');

async function readLastLine(name) {
  var line = '';
  var chunkSize = 200 // how many characters to read from the end of file
  const fd = await open(name);
  const st = await fd.stat();
  const buf = Buffer.alloc(chunkSize);
  const len = buf.length;
  const { bytesRead, buffer } = await fd.read(buf, 0, len, st.size - len)
  for (let i = len - 1; i > -1; i--) {
    const isEol = buffer[i] === 0x0a // 0x0a == '\n'
    const isCtrl = buffer[i] < 0x20 // 0-31 are ASCII control characters
    if (isEol && line.length > 0) {
      break;
    } else if (!isCtrl && !isEol) {
      line = String.fromCharCode(buffer[i]) + line;
    }
  }
  fd.close();
  return line;
}

try {
  const line = await readLastLine(fileName)
  console.log(line);
} catch (err) {
  console.error(err);
}

Old 2014 answer

    var fs = require('fs');
    var path = require('path');
    var fileName = path.join(__dirname, 'test.txt');
    var readLastLine = function(name, callback) {
        fs.stat(name, function(err, stat) {
            fs.open(name, 'r', function(err, fd) {
                if(err) throw err;
                var i = 0;
                var line = '';
                var readPrevious = function(buf) {
                    fs.read(fd, buf, 0, buf.length, stat.size-buf.length-i, function(err, bytesRead, buffer) {
                        if(err) throw err;
                        line = String.fromCharCode(buffer[0]) + line;
                        if (buffer[0] === 0x0a) { //0x0a == '\n'
                            callback(line);
                        } else {
                            i++;
                            readPrevious(new Buffer(1));
                        }
                    });
                }
                readPrevious(new Buffer(1));
            });
        });
    }
    readLastLine(fileName, function(line) {
        console.log(line);
    });

Upvotes: 9

mak
mak

Reputation: 13405

Regular file

Read the file like a regular file, split the file contents into lines, take the last line, split by a comma and take the last part.

var fs = require('fs'); // file system module

fs.readFile('/path/to/file.csv', 'utf-8', function(err, data) {
    if (err) throw err;

    var lines = data.trim().split('\n');
    var lastLine = lines.slice(-1)[0];

    var fields = lastLine.split(',');
    var audioFile = fields.slice(-1)[0].replace('file:\\\\', '');

    console.log(audioFile);
});

File System module documentation

CSV parser

You can also use the node-csv-parser module.

var fs = require('fs');
var csv = require('csv');

csv()
 .from.stream(fs.createReadStream('/path/to/file.csv'))
 .to.array(function(data, count) {
    var lastLine = data.slice(-1)[0];
    var audioFile = lastLine.slice(-1)[0].replace('file:\\\\', '');
    console.log(audioFile);
  });

Upvotes: 9

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