day-2.js

const input2 = [
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];

let position = input2.reduce((pos, thisCommand) => {
  if (thisCommand[0] === "forward") {
    pos.horizontal += thisCommand[1];
  } else if (thisCommand[0] === "down") {
    pos.depth += thisCommand[1];
  } else { // up
    pos.depth -= thisCommand[1];
  }
  return pos;
}, {
  horizontal: 0,
  depth: 0
});
console.log(position.horizontal * position.depth);

// down X increases your aim by X units.
// up X decreases your aim by X units.
// forward X does two things:
// It increases your horizontal position by X units.
// It increases your depth by your aim multiplied by X.

let position2 = input2.reduce((pos, thisCommand) => {
  if (thisCommand[0] === "down") {
    pos.aim += thisCommand[1];
  } else if (thisCommand[0] === "up") {
    pos.aim -= thisCommand[1];
  } else { // forward
    pos.horizontal += thisCommand[1];
    pos.depth += pos.aim * thisCommand[1];
  }
  return pos;
}, {
  horizontal: 0,
  depth: 0,
  aim: 0
});
console.log(position2.horizontal * position2.depth);