Writing SOLID code

Reference

• http://williamdurand.fr/2013/07/30/from-stupid-to-solid-code/
• http://williamdurand.fr/2013/06/03/object-calisthenics/

Dependency Inversion Principle

Abstractions should not depend upon details

Details should depend upon abstractions.

Dependency Inversion Principle

Dependency Inversion Principle

1. One level of indentation per method

Too many levels of indentation = 

bad readability

Hint: Split the method into several smaller ones.

// Before
function validateCollection() {
    this.collection.forEach(function (item) {
        item.forEach(function (property) {
            if (typeof property !== 'string') {
                throw new Error('invalid');
            }
        });
    });

    return true;
}

// After
function validateCollection() {
    this.collection.forEach(function (item) {
        this.validateItem(item);
    });
}

function validateItem(item) {
    item.forEach(function (property) {
        this.validateProperty(property);
    });
}

function validateProperty(property) {
    if (typeof property !== 'string') {
        throw new Error('invalid');
    }
}

2. Don't use the "else" keyword

Nested and branching conditionals =

bad readbility

Hint: Early return.

function validateProperty(property) {
    if (property.isString()) {
        if (property.equals('invalid')) {
            return false;
        } else {
            return true;
        }
    } else {
        throw new Error('Not a string');
    }
}

function validateProperty() {
    if (!property.isString()) {
        throw new Error('Not a string');
    }

    if (property.equals('invalid')) {
        return false;
    }

    return true;
}

2. Don't use the "else" keyword

3. Wrap all primitives and strings

If it has a behaviour, encapsulate.

3. Wrap all primitives and strings

var date = '2016-05-31';
// 120 bucks
var money = 120;
// mph
var velocity = 80;

var date = new Date('2016-05-31');
var money = new Money(120, 'USD');
var velocity = new Velocity(80, 'mph');
velocity.increaseBy(20);
velocity.decreaseBy(150); // --> error, cannot be negative

4. First class collections

Encapsulate a set of elements in an object and add specific behaviours (methods).

First class collections

// Array
var userData = [
    // ...
];
var user = null;

for (var i = 0; i < userData.length; i++) {
    if (userData[i].username === 'testuser') {
        user = userData[i];
        break;
    }
}

// Collection
var users = new UserCollection(userData);

users.count();
users.findByUsername('testuser');

5. One dot per line

• Law of Demeter: Don't talk to strangers.
• Doesn't apply for "fluent interfaces".

var totalMembers = 0;
users.getUser('testuser').getGroups().forEach(function(group) {
     totalMembers += group.getMembers().count();
});

// User
function getTotalGroupMembers() {
    return this.getGroups().getTotalMembers();
}

// Group collection
function getTotalMembers() {
    var number = 0;
    this.forEach(function(group) {
        number += group.getMemberCount();
    });
    
    return number;
}

// Group
function getMemberCount() {
    return this.members.count();
}

5. One dot per line

9. No getters and setters

Implement behaviours instead of exposing class data through getters/setters --> better encapsulation.

9. No getters and setters

var account = new Account(120);
var addedAmount = 30;

var currentAmount = account.getBallance();
account.setBallance(currentAmount + addedAmount);

// instead:
account.addAmmount(30);