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

Example Input:

MMMSXXMASM
MSAMXMSMSA
AMXSXMAAMM
MSAMASMSMX
XMASAMXAMM
XXAMMXXAMA
SMSMSASXSS
SAXAMASAAA
MAMMMXMMMM
MXMXAXMASX

Part 1 Solution:

const fs = require('fs');

const data = fs.readFileSync('./input.txt', 'utf8');

const grid = data.trim().split('\n');

const gridHeight = grid.length
const gridWidth = grid[0].length

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

let matches = 0;

for(y=0; y < gridHeight; y++) {
    for (x=0; x < gridWidth; x++) {
        if (grid[y][x] !== 'X') continue;

        matches += directions.reduce((sum, direction) => {
            let word = 'X';
            let curX = x, curY = y;

            for(i=0; i<3; i++) {
                curX += direction.x
                curY += direction.y

                if (!(curX < 0 || curX >= gridWidth || curY < 0 || curY >= gridHeight)) {
                    word += grid[curY][curX]
                }
            }

            return sum + (word === 'XMAS' ? 1 : 0);
        }, 0)
    }
}

console.log(matches)

Part 2 Solution:

const fs = require('fs');

const data = fs.readFileSync('./input.txt', 'utf8');

const grid = data.trim().split('\n');

const gridHeight = grid.length
const gridWidth = grid[0].length

let matches = 0;

function getChar(x, y) {
    if (!(x < 0 || x >= gridWidth || y < 0 || y >= gridHeight)) {
        return grid[y][x];
    }
}

for(y=0; y < gridHeight; y++) {
    for (x=0; x < gridWidth; x++) {
        if (grid[y][x] !== 'A') continue;

        const word1 = getChar(x - 1, y + 1) + 'A' + getChar(x + 1, y - 1);
        const word2 = getChar(x + 1, y + 1 ) + 'A' + getChar(x - 1, y - 1);

        if ((word1 === 'MAS' || word1 === 'SAM') && (word2 === 'MAS' || word2 === 'SAM')) matches++;
    }
}

console.log(matches)