For a description of this problem, please check the Advent of Code website.

Example Input:

....#.....
.........#
..........
..#.......
.......#..
..........
.#..^.....
........#.
#.........
......#...

Part 1 Solution:

const fs = require('fs');

const field = fs.readFileSync('./example-input.txt', 'utf8').trim().split("\n").map(row => [...row]);

const directions = [
    { x: 0, y: -1 },
    { x: 1, y: 0 },
    { x: 0, y: 1 },
    { x: -1, y: 0 },
];

let x, y;

for (let i = 0; i < field.length; i++) {
    const colIndex = field[i].indexOf('^');
    if (colIndex !== -1) {
        y = i;
        x = colIndex;
        break;
    }
}

let currentDirection = directions[0];

const visitedLocations = new Set();
visitedLocations.add(`${x},${y}`);

let visitedCount = 0;

const turn = () => {
    currentDirection = directions[(directions.indexOf(currentDirection) + 1) % 4];
};

while (
    y + currentDirection.y >= 0 &&
    y + currentDirection.y < field.length &&
    x + currentDirection.x >= 0 &&
    x + currentDirection.x < field[0].length
) {
    while (
        y + currentDirection.y >= 0 &&
        y + currentDirection.y < field.length &&
        x + currentDirection.x >= 0 &&
        x + currentDirection.x < field[0].length &&
        field[y + currentDirection.y][x + currentDirection.x] === '#'
    ) {
        turn();
    }

    x += currentDirection.x;
    y += currentDirection.y;

    const key = `${x},${y}`;
    if (!visitedLocations.has(key)) {
        visitedLocations.add(key);
        visitedCount++;
    }
}

console.log(visitedCount);

Part 2 Solution:

const fs = require('fs');

const field = fs.readFileSync('./example-input.txt', 'utf8').trim().split("\n").map(row => [...row]);

const directions = [
    { x: 0, y: -1 },
    { x: 1, y: 0 },
    { x: 0, y: 1 },
    { x: -1, y: 0 },
];

function checkLoop() {
    let x, y;

    for (let i = 0; i < field.length; i++) {
        const colIndex = field[i].indexOf('^');
        if (colIndex !== -1) {
            y = i;
            x = colIndex;
            break;
        }
    }

    let currentDirection = directions[0];

    const visitedLocations = new Set();
    
    const turn = () => {
        currentDirection = directions[(directions.indexOf(currentDirection) + 1) % 4];
    };
    
    while (
        y + currentDirection.y >= 0 &&
        y + currentDirection.y < field.length &&
        x + currentDirection.x >= 0 &&
        x + currentDirection.x < field[0].length
    ) {
        while (
            y + currentDirection.y >= 0 &&
            y + currentDirection.y < field.length &&
            x + currentDirection.x >= 0 &&
            x + currentDirection.x < field[0].length &&
            field[y + currentDirection.y][x + currentDirection.x] === '#'
        ) {
            turn();
        }
    
        x += currentDirection.x;
        y += currentDirection.y;
    
        const key = `${x},${y},${currentDirection.x},${currentDirection.y}`;
        if (!visitedLocations.has(key)) {
            visitedLocations.add(key);
        } else {
            return true
        }
    }

    return false
}
let loopCount = 0;

// This could be optimized by only checking squares visited in part 1
for (let y = 0; y < field.length; y++) {
    for (let x = 0; x < field.length; x++) {
        if(field[y][x] === '.') {
            field[y][x] = '#'
            if(checkLoop()) {
                loopCount++
            }
            field[y][x] = '.'
        }
    }
}

console.log(loopCount);