advent_of_code_2.js

const input = document.children[0].textContent.split('\n').filter(x => x.length).map(movement => movement.split(' '))

input.reduce((acc, currentTuple, index, array) => {
    switch(currentTuple[0]) {
        case "forward": acc[0] = acc[0] + parseInt(currentTuple[1]); break;
        case "up" : acc[1] = acc[1] + parseInt(currentTuple[1]); break;
        case "down": acc[1] = acc[1] - parseInt(currentTuple[1]); break;
}
return acc;
}, [0, 0]);