Final code for https://youtu.be/ncyzY-5MyB8

2_gol_oop.js

import expect from 'expect.js'

class GameOfLifeBoard {
    alvie = {};

    isAlive(cell) {
        return !!this.alvie[cell];
    }

    spawn(cell) {
        this.alvie[cell] = cell
        return this;
    }

    list() {
        return Object.values(this.alvie);
    }

}

class GameOfLife {
    board = new GameOfLifeBoard();

    isAlive(cell) {
        return this.board.isAlive(cell);
    }

    spawn(cell) {
        this.board.spawn(cell)
    }

    evolve() {
        this.board = this.board
            .list()
            .flatMap((cell) => cell.neighbours())
            .filter(cell => this.shouldBeAlive(cell))
            .reduce((next, cell) => next.spawn(cell), new GameOfLifeBoard());
    }

    shouldBeAlive(cell) {
        let count = this.countNeighbours(cell);
        return (count === 2 && this.isAlive(cell)) || count === 3;
    }

    countNeighbours(cell) {
        return cell.neighbours()
            .filter(cell => this.isAlive(cell))
            .length
    }
}

class Cell {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }

    toString() {
        return `(${this.x},${this.y})`
    }

    neighbours() {
        return [
            { x: -1, y: -1},
            { x: -1, y:  0},
            { x: -1, y:  1},
            { x:  0, y: -1},
            { x:  0, y:  1},
            { x:  1, y: -1},
            { x:  1, y:  0},
            { x:  1, y:  1},
        ]
            .map(delta => new Cell(this.x + delta.x, this.y + delta.y))
    }
}

describe("GameOfLife", () => {
    let game = null;
    const cell = (x, y) => new Cell(x, y);
    const expectDead = (cell) => expect(game.isAlive(cell)).to.be(false);
    const expectAlive = (cell) => expect(game.isAlive(cell)).to.be(true);

    beforeEach(() => {
       game = new GameOfLife();
    });

    specify("that an empty world has dead (0,0) cell", () => {
        expectDead(cell(0,0));
    });

    specify("that a (0,0) cell can be spawned", () => {
        game.spawn(cell(0,0));
        expectAlive(cell(0,0));
    });

    specify("that a (3,5) cell can be spawned independently of (0,0) cell", () => {
        game.spawn(cell(3, 5));
        expectDead(cell(0,0));
    });

    specify("that empty world has dead (0,0) cell after being evolved", () => {
        game.evolve();
        expectDead(cell(0,0));
    });

    specify("that lonely (0,0) cell dies when evolved", () => {
        game.spawn(cell(0,0));
        game.evolve();
        expectDead(cell(0,0));
    });

    specify("that (0,0) cell with single neighbour (-1, -1) dies", () => {
        game.spawn(cell( 0, 0));
        game.spawn(cell(-1,-1));
        game.evolve();
        expectDead(cell(0,0));
    });

    [
        cell(-1, 0),
        cell(-1, 1),
        cell( 0,-1),
        cell( 0, 1),
        cell( 1,-1),
        cell( 1, 0),
        cell( 1, 1)
    ].forEach(neighbourCell => {
        specify(`that (0,0) cell with two neighbours (-1, -1) and ${neighbourCell} stays alive`, () => {
            game.spawn(cell( 0, 0));
            game.spawn(cell(-1,-1));
            game.spawn(neighbourCell);
            game.evolve();
            expectAlive(cell(0,0));
        });
    });

    specify("that (0,0) cell with three neighbours (-1, -1), (-1, 0) and (-1,1) stays alive", () => {
        game.spawn(cell( 0, 0));
        game.spawn(cell(-1,-1));
        game.spawn(cell(-1, 0));
        game.spawn(cell(-1, 1));
        game.evolve();
        expectAlive(cell(0,0));
    });

    specify("that dead (0,0) cell with three neighbours (-1, -1), (-1, 0) and (-1,1) spawn", () => {
        game.spawn(cell(-1,-1));
        game.spawn(cell(-1, 0));
        game.spawn(cell(-1, 1));
        game.evolve();
        expectAlive(cell(0,0));
    });

    specify("that dead (0,0) cell with two neighbours (0, -1) and (1,1) stays dead", () => {
        game.spawn(cell( 0,-1));
        game.spawn(cell( 1, 1));
        game.evolve();
        expectDead(cell(0,0));
    });

    specify("that (3,5) cell with two neighbours stays alive", () => {
        game.spawn(cell(3,5));
        game.spawn(cell(3,4));
        game.spawn(cell(4,6));
        game.evolve();
        expectAlive(cell(3,5));
    });

    specify("that (-2,8) cell with two neighbours stays alive", () => {
        game.spawn(cell(-2,8));
        game.spawn(cell(-1,7));
        game.spawn(cell(-3,9));
        game.evolve();
        expectAlive(cell(-2,8));
    });

});