convays-game-of-life.js

const currentField = ['0:1', '1:1', '2:1'];

function parseItem(item) {
    return item.split(':').map(Number); // [ x, y ]
}

function countNeighbours(x, y) {
    let sum = 0;

    for(var t = -1; t <= 1; t++){
        for(var r = -1; r <= 1; r++){
            if (!t && !r) {
                continue;
            }

            if (currentField.indexOf(`${x + t}:${y + r}`) !== -1) {
                sum++;
            }
        }
    }

    return sum;
}

function cellIsAlive(count, state) {
    if (count < 2  || count > 3) {
        return false;
    }

    if (count === 2) {
        return Boolean(state);
    }

    return true;
}

function step(array) {
    const nextField = [];

    const parsedArray = array.map(parseItem);

    for(let i = -1; i <= 1; i++) {
        for(let j = -1; j <= 1; j++) {
            parsedArray.forEach(function(point) { // [x, y]
                const nextX = point[0] + i;
                const nextY = point[1] + j;
                const isCurrentAlive = array.indexOf(`${nextX}:${nextY}`) !== -1;

                if (cellIsAlive(countNeighbours(nextX, nextY), isCurrentAlive)) {
                    if (nextField.indexOf(`${nextX}:${nextY}`) === -1) {
                        nextField.push(`${nextX}:${nextY}`);
                    }
                }
            })
        }
    } 

    return nextField;
}

console.log(step(currentField));