Clean Code Cookbook

Chapter 24. Types

Types are essentially assertions about a program. And I think it’s valuable to have things be as absolutely simple as possible, including not even saying what the types are.

Dan Ingalls, Coders at Work: Reflections on the Craft of Programming

24.0 Introduction

Types are the most important concept in classification languages. This is true for static and strongly typed languages and also for dynamically typed ones. Dealing with them is not easy and you have many flavors, from very restrictive ones to lazier ones.

24.1 Removing Type Checking

Problem

You type-check your arguments.

Solution

Trust your collaborators. Don’t check who they are. Ask them to do it instead.

Discussion

Avoid kind(), isKindOf(), instance(), getClass(), typeOf(), etc., and don’t use reflection and metaprogramming for domain objects (see Chapter 23, “Metaprogramming”). Avoid checking for undefined. Use complete objects (see Recipe 3.7, “Completing Empty Constructors”), and avoid nulls (see Recipe 15.1, “Creating Null Objects”) and setters. Favor immutability and you will never have undefined types or accidental ifs.

Here are examples of type checking:

if (typeof(x) === 'undefined') {
    console.log('variable x is not defined');  
}

function isNumber(data) {
  return (typeof data === 'number');
}

And here’s a complete type-checking example:

function move(animal) {
  if (animal instanceof Rabbit) {
      animal.run()
  }
  if (animal instanceof Seagull) {
      animal.fly()
  }
}

class Rabbit {
  run() {
    console ...