part-two.js

let initial_state = {
    boss: 51,
    damage: 9,

    hp: 50,
    mana: 500,
    armor: 0,
    manaSpent: 0,

    shield: 0,
    poison: 0,
    recharge: 0,

    move: 'player',
};

const MISSILE = 'missile';
const DRAIN = 'drain';
const SHIELD = 'shield';
const POISON = 'poison';
const RECHARGE = 'recharge';
const spells = {
    [MISSILE]: { cost: 53, damage: 4, heal: 0 },
    [DRAIN]: { cost: 73, damage: 2, heal: 2 },
    [SHIELD]: { cost: 113, duration: 6 },
    [POISON]: { cost: 173, duration: 6 },
    [RECHARGE]: { cost: 229, duration: 5 },
};

let min = Number.MAX_SAFE_INTEGER;
let moves = [initial_state];
let state;

while ((state = moves.pop())) {
    // On hard mode, player loses 1 HP each turn
    if (state.move === 'player') {
        state.hp--;
    }

    // Check effects and apply them if they are greater than 0
    // This ends up reducing these effects below zero in some cases but it doesn't end up affecting anything
    state.armor = state.shield-- > 0 ? 7 : 0;

    if (state.poison-- > 0) {
        state.boss -= 3;
    }

    if (state.recharge-- > 0) {
        state.mana += 101;
    }

    // If we've lost all our HP, or we've spent more mana than our current minimum we've found, exit this path
    if (state.hp <= 0 || state.manaSpent >= min) {
        continue;
    }

    // If our boss is dead, see if our mana spent is less than our current minimum
    if (state.boss <= 0) {
        min = Math.min(min, state.manaSpent);
        continue;
    }

    if (state.move === 'boss') {
        // Switch to player for next move
        state.move = 'player';

        // Boss always does at least 1 damage
        state.hp -= Math.max(1, state.damage - state.armor);

        moves.push(state);

        // console.log('  Boss { ' + Object.keys(state).map(k => `${k === 'move' ? k + ': "' + state[k] + '"' : k + ': ' + state[k]}`).join(', ') + ' }');
    } else {
        // Switch to boss for next move
        state.move = 'boss';

        // We are on the player, loop through all the spells and push them to our possible moves
        for (let spell in spells) {
            let { cost, damage, heal, duration } = spells[spell];

            // If this costs more than the mana we have, skip it
            // Note that if we spend all out mana, this effectively kills the branch because
            // all spells will be skipped and we'll never `push` a new state onto our moves array
            if (cost >= state.mana) {
                continue;
            }

            // Clone our current state
            let new_state = Object.assign({}, state);

            new_state.mana -= cost;
            new_state.manaSpent += cost;

            if (spell === MISSILE || spell === DRAIN) {
                // Missle and Drain have no effect, they are instant spells
                new_state.boss -= damage;
                new_state.hp += heal;
            } else {
                // If we've already activated this effect, skip it
                if (new_state[spell] > 0) {
                    continue;
                }

                new_state[spell] = duration;
            }

            moves.push(new_state);

            // console.log('Player { ' + Object.keys(state).map(k => `${k === 'move' ? k + ': "' + n[k] + '"' : k + ': ' + n[k]}`).join(', ') + ' }');
        }
    }
}

console.log(min);