Advent of Code Day 2 Solver

First we have to process our input and extract the useful info.  We split line by line to get each password on it's own and iterate through all our lines.  For each line we then use some string splitting and array destructuring to get the valuable information into properly named variables.  We want to extract the two integers (as lower and upper bounds), the main character we will be testing against, and the actuall password string each into their own named variables.

Then we check through each character of the main password string, counting the number that match our test char.  If that count falls within our lower and upper bounds we have found a valid password and can add it to our numValid running total.
          const fs = require('fs');
const input = fs.readFileSync('day2/node/input.txt', "utf8").split('\n').map(el => el.split(' '));

let numValid = 0;

for(line of input){
    const [rule, char, password] = line;
    const [lowerbound, upperbound] = rule.split('-');
    const counterChar = char[0];

    let count = 0;
    for(character of password.split('')){
        if(character === counterChar){
            ++count;
        }
    }

    if( lowerbound <= count && count <= upperbound){
        ++numValid;
    }
}

console.log(numValid);
      

Again for part two we start with transforming our input and extracting the same relevant information.  This time we will check only specific indices within the password string (making sure to subtract one since an array's first index is zero!).  If at least one of the characters at those indices matches the target char, we check that the other doesn't also match, and then can add it to our tally of valid passwords satisfying the new password rules.
          const fs = require('fs');
const input = fs.readFileSync('day2/node/input.txt', "utf8").split('\n').map(el => el.split(' '));

let numValid = 0;

for(line of input){
    let [rule, char, password] = line;
    const [position1, position2] = rule.split('-');
    char = char[0];
    password = password.split('');
    const str1 = password[position1 - 1];
    const str2 = password[position2 - 1]; 
  
    if((str1 === char || str2 === char) && str1 !== str2){
        ++numValid;
    }

}

console.log(numValid);