A variation on http://martinfowler.com/articles/refactoring-video-store-js/

video-store-refactor.js

let AmountCalculator = {
    'new'(days) {
        return days * 3;
    },
    'regular'(days) {
        let amount = 2;
        return days > 2 ? (amount + (days - 2) * 1.5) : amount;
    },
    'childrens'(days) {
        let amount = 1.5;
        return days > 3 ? (amount + (days - 3) * 1.5) : amount;
    }
};

let BonusCalculator = {
    'default': 1,
    'new'(days) {
        return days > 1 ? this.default + 1 : this.default;
    },
    'regular'(days) {
        return this.default;
    },
    'childrens'(days) {
        return this.default;
    }
};

function calculatePerMovie(days, movieId, movies) {    
    return {
        'id': movieId,
        'amount': AmountCalculator[movies[movieId].code](days),
        'bonus': BonusCalculator[movies[movieId].code](days)
    };
}

function calculate(customer, movies) {

    let name = customer.name;
    let perMovie = [];
    let total = {
        amount: 0,
        bonus: 0
    };

    for (let r of customer.rentals) {
        let movieCalculation = calculatePerMovie(r.days, r.movieID, movies);
        total.amount += movieCalculation.amount;
        total.bonus += movieCalculation.bonus;
        perMovie.push(movieCalculation);
    }

    return { name, perMovie, total };
}

function createTextOutput(result, movies) {

    let output = '';
    let amount = 0;
    let bonus = 0;

 	output += `Rental Record for ${result.name}\n`;

    for (let r of result.perMovie) {
        output += `\t${movies[r.id].title}\t${r.amount}\n`;
        amount += r.amount;
        bonus += r.bonus;
    }

    output += `Amount owed is ${result.total.amount}\n`;
    output += `You earned ${result.total.bonus} frequent renter points\n`;

    return output;
}

function createHtmlOutput(result, movies) {

    let output = `<h1>Rental Record for <em>${result.name}</em></h1>\n`;
 
    output += '<table>\n';
    
    for (let r of result.perMovie) {
        output += `  <tr><td>${movies[r.id].title}</td><td>${r.amount}</td></tr>\n`;
    }
    
    output += '</table>\n';
    output += `<p>Amount owed is <em>${result.total.amount}</em></p>\n`;
    output += `<p>You earned <em>${result.total.bonus}</em> frequent renter points</p>\n`;

    return output;
}

let customer = {
    "name": "martin",
    "rentals": [
        { "movieID": "F001", "days": 3 },
        { "movieID": "F002", "days": 1 }
    ]
};

let movies = {
    "F001": { "title": "Ran", "code": "regular" },
    "F002": { "title": "Trois Couleurs: Bleu", "code": "regular" }
};

let result = calculate(customer, movies);

console.log(createTextOutput(result, movies));
console.log(createHtmlOutput(result, movies));