How to Use JavaScript to Read Local Text File and Read Line by Line

How to use Javascript to read local text file and read line by line?

Without jQuery:

const $output = document.getElementById('output')
document.getElementById('file').onchange = function() {
  var file = this.files[0];

  var reader = new FileReader();
  reader.onload = function(progressEvent) {
    // Entire file
    const text = this.result;
    $output.innerText = text

    // By lines
    var lines = text.split('\n');
    for (var line = 0; line < lines.length; line++) {
      console.log(lines[line]);
    }
  };
  reader.readAsText(file);
};

<input type="file" name="file" id="file">
<div id='output'>
  ...
</div>

How to read a local text file in the browser?

You need to check for status 0 (as when loading files locally with XMLHttpRequest, you don't get a status returned because it's not from a Webserver)

function readTextFile(file)
{
    var rawFile = new XMLHttpRequest();
    rawFile.open("GET", file, false);
    rawFile.onreadystatechange = function ()
    {
        if(rawFile.readyState === 4)
        {
            if(rawFile.status === 200 || rawFile.status == 0)
            {
                var allText = rawFile.responseText;
                alert(allText);
            }
        }
    }
    rawFile.send(null);
}

And specify file:// in your filename:

readTextFile("file:///C:/your/path/to/file.txt");

How to read online file line by line?

You can use the Fetch API and read the response as text and iterate line by line

(async () => {
  const response = await fetch("https://xn--qucu-hr5aza.cc/files/M%E1%BB%99t%20h%E1%BB%87%20th%E1%BB%91ng%20ni%E1%BB%81m%20tin/Node%20list");
  const data = await response.text();
  const lines = data.split("\n");
  console.log(lines)
})();

Read a file one line at a time in node.js?

Since Node.js v0.12 and as of Node.js v4.0.0, there is a stable readline core module. Here's the easiest way to read lines from a file, without any external modules:

const fs = require('fs');
const readline = require('readline');

async function processLineByLine() {
  const fileStream = fs.createReadStream('input.txt');

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity
  });
  // Note: we use the crlfDelay option to recognize all instances of CR LF
  // ('\r\n') in input.txt as a single line break.

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine();

Or alternatively:

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('file.in')
});

lineReader.on('line', function (line) {
  console.log('Line from file:', line);
});

The last line is read correctly (as of Node v0.12 or later), even if there is no final \n.

UPDATE: this example has been added to Node's API official documentation.

Read text file line by line and execute a function?

You can use the ReadLine module to read files line by line. You can append a list of the test results and then do something with them when the file is completely read:

const readline = require('readline');
const fs = require('fs');
const inputFileName = './testfile.txt';

const readInterface = readline.createInterface({
    input: fs.createReadStream(inputFileName),
});

let testResults = [];
readInterface.on('line', line => {
    testResult = test(line);
    console.log(`Test result (line #${testResults.length+1}): `, testResult);
    testResults.push({ input: line, testResult } );
});

// You can do whatever with the test results here.
readInterface.on('close', () => {
    console.log("Test results:", testResults);
});

function test(str){

    let regex = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/; // email regex

    str = str.split(","); 

    // string should be of length 3 with str[1] number of length 7
    if(str && str.length === 3 && Number(str[1]) && str[1] ) {

        let temp = str[0].split("-");

        // check for 85aecb80-ac00-40e3-813c-5ad62ee93f42 separately.
        if(temp && temp.length === 5 &&  /[a-zA-Z\d]{8}/.test(temp[0]) &&  /[a-zA-Z\d]{4}/.test(temp[1]) &&  /[a-zA-Z\d]{4}/.test(temp[2]) &&  /[a-zA-Z\d]{4}/.test(temp[3]) &&  /[a-zA-Z\d]{12}/.test(temp[4])){

            // email regex
            if(regex.test(str[2])) {
                return true;
            } else {
                return false;
            }
        } else { 
            return false
        }
    } else {
        return false;
    }
}

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