simple_game.js

// A simple game engine

// #1: The state of the game

// Aliens starting positions
const aliens = [{x: 10, y: 10}, {x:20, y:5}, {x: 25, y:15}]

// Ship info
const ship = { x: 5, direction: 0 }

// Size of the game screen
const width = 30
const height = 15

// Message to display
let message = 'A or D to move. Q to quit'

let score = 0;

// #2: Rendering

const draw = () => {
    // Use a buffer to avoid flickering
    //
    // The first character in the buffer is
    // a special code that clears the screen
    const buffer = ['\033c', `Score: ${score}    ${message}\n`]

    // Loop `y` from top to bottom
    for(let y = height; y >= 0 ; y--){

        // Loop `x` from left to right
        for(let x = 0; x < width; x++){

            let out = ' ';

            // Draw some stars at pseudo-random locations
            if ((x * y) % 11 == 3) {
                out = '.'
            }

            // Draw the ship
            if (y == 0 && x == ship.x) {
                out = 'W';
            }

            // Draw the aliens
            for (let alien of aliens) {
                if(alien.x == x && alien.y == y){
                    out = '@';
                }
            }

            buffer.push(out)
        }

        if(y !== 0) {
            buffer.push('\n'); // next line
        }
    }

    // write the buffer to the screen
    process.stdout.write(buffer.join(''))
}

// #3: The game logic and physics engine

// make the aliens fall
const fall = () => {
    for(let alien of aliens) {
        alien.y--

        // If an alien touches the ground
        if (alien.y < 0) {

            // If it touched the ship, game over!
            if (alien.x === ship.x) {
                console.log('Game Over!')
                process.exit(0);
            }

            // Give the player one point
            score++;

            // Move it back to the top
            alien.y = height;

            // Add another random alien
            aliens.push({
                x: randomBetween(0, width),
                y: randomBetween(height - 5, height)
            })
        }
    }
}

// Move the ship
const move = () => {
    ship.x += ship.direction;
    if (ship.x < 0) ship.x = 0;
    if (ship.x >= width) ship.x = width - 1;
    ship.direction = 0;
}

// #4: handling player input

process.stdin.setRawMode(true);
process.stdin.on('data', (input) => {
    const code = input[0];
    switch (code) {

    case 3: // Ctrl+C
    case 113: // 'q'
        process.exit(0);

    case 97: // 'a'
        ship.direction = -1;
        break;

    case 100: // 'd'
        ship.direction = 1;
        break;

    default:
        message = `Unkown character code: ${code}`
    }
});

const tick = () => {
    fall();
    move();
    draw();
}

// The game loop
const tickAndContinue = () => {
    tick();
    setTimeout(tickAndContinue, 250); // four frames per second
}

// Run the game
tickAndContinue()

// Utility functions
const randomBetween = (a, b) => {
    return Math.floor(a + Math.random() * (b - a + 1))
}

Run using NodeJS

node simple_game.js