Question about Find string and delete line - Node.JS - node.js

Question

Find string and delete line - Node.JS
var fs = require('fs')
fs.readFile('shuffle.txt', {encoding: 'utf-8'}, function(err, data) {
if (err) throw error;
let dataArray = data.split('\n'); // convert file data in an array
const searchKeyword = 'user1'; // we are looking for a line, contains, key word 'user1' in the file
let lastIndex = -1; // let say, we have not found the keyword
for (let index=0; index<dataArray.length; index++) {
if (dataArray[index].includes(searchKeyword)) { // check if a line contains the 'user1' keyword
lastIndex = index; // found a line includes a 'user1' keyword
break;
}
}
dataArray.splice(lastIndex, 1); // remove the keyword 'user1' from the data Array
// UPDATE FILE WITH NEW DATA
// IN CASE YOU WANT TO UPDATE THE CONTENT IN YOUR FILE
// THIS WILL REMOVE THE LINE CONTAINS 'user1' IN YOUR shuffle.txt FILE
const updatedData = dataArray.join('\n');
fs.writeFile('shuffle.txt', updatedData, (err) => {
if (err) throw err;
console.log ('Successfully updated the file data');
});
});
This link explains how to find a string and delete lines but only delete one user1 at the time. I have many lines with user1, how can I delete all:
john
doe
some keyword
user1
last word
user1
user1
Also the opposite. How can I delete all the lines and leave only the user1 lines?

Answer 1

I would make use of the Array.filter() function.
Basically, you call filter() on an array, and define a callback function that checks every item of that array.
If the checking function returns true for a particular item, keep that item - put it in a new array
If the checking function returns false, do not put the item in the new array
So, in your case, once you have read all the lines into an array (up to line 6 in your code), use the filter function:
// Delete all instances of user1
let newDataArray = dataArray.filter(line => line !== "user1")
// Delete everything except user1
let newDataArray = dataArray.filter(line => line === "user1")
// Delete any lines that have the text 'user1' somewhere inside them
let newDataArray = dataArray.filter(line => !line.includes("user1"))
Then, just like you have done in your code, use the join() function on newDataArray() and write the file.
To rewrite your code,
var fs = require('fs')
fs.readFile('shuffle.txt', {encoding: 'utf-8'}, function(err, data) {
if (err) throw error;
let dataArray = data.split('\n'); // convert file data in an array
const searchKeyword = 'user1'; // we are looking for a line, contains, key word 'user1' in the file
// Delete all instances of user1
let newDataArray = dataArray.filter(line => line !== searchKeyword)
// UPDATE FILE WITH NEW DATA
const updatedData = newDataArray.join('\n');
fs.writeFile('shuffle.txt', updatedData, (err) => {
if (err) throw err;
console.log ('Successfully updated the file data');
});
});