Composing Software
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Composing Software

An Exploration of Functional Programming and Object Composition in JavaScript

About the Book

All software design is composition: the act of breaking complex problems down into smaller problems and composing those solutions. Most developers have a limited understanding of compositional techniques. It's time for that to change.

In "Composing Software", Eric Elliott shares the fundamentals of composition, including both function composition and object composition, and explores them in the context of JavaScript. The book covers the foundations of both functional programming and object oriented programming to help the reader better understand how to build and structure complex applications using simple building blocks.

You'll learn:

  • Functional programming
  • Object composition
  • How to work with composite data structures
  • Closures
  • Higher order functions
  • Functors (e.g.,
  • Monads (e.g., promises)
  • Transducers
  • Lenses

All of this in the context of JavaScript, the most used programming language in the world. But the learning doesn't stop at JavaScript. You'll be able to apply these lessons to any language. This book is about the timeless principles of software composition and its lessons will outlast the hot languages and frameworks of today. Unlike most programming books, this one may still be relevant 20 years from now.

This book began life as a popular blog post series that attracted hundreds of thousands of readers and influenced the way software is built at many high growth tech startups and fortune 500 companies.

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  • Categories

    • Computers and Programming
    • Functional Programming
    • JavaScript
    • Software
    • Software Engineering
    • Software Architecture
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About the Author

Eric Elliott
Eric Elliott

Eric Elliott is a distributed systems expert and author of the books, “Composing Software” and “Programming JavaScript Applications”. As co-founder of, he teaches developers the skills they need to work remotely and embrace work/life balance. He builds and advises development teams for crypto projects, and has contributed to software experiences for Adobe Systems, Zumba Fitness, The Wall Street Journal, ESPN, BBC, and top recording artists including Usher, Frank Ocean, Metallica, and many more.

He enjoys a remote lifestyle with the most beautiful woman in the world.

Table of Contents

  • Thank You
  • Composing Software: An Introduction
    • You Compose Software Every Day
    • Conclusion
  • The Dao of Immutability (The Way of the Functional Programmer)
    • Forward
  • The Rise and Fall and Rise of Functional Programming (Composable Software)
    • The Rise of Functional Programming
    • The Fall of Functional Programming
    • The Rise of Functional Programming
    • Functional Programming Has Always Been Alive and Well
  • Why Learn Functional Programming in JavaScript?
  • Pure Functions
    • What is a Function?
    • Mapping
    • Pure Functions
    • The Trouble with Shared State
    • Same Input, Same Output
    • No Side Effects
    • Conclusion
  • What is Functional Programming?
    • Pure Functions
    • Function Composition
    • Shared State
    • Immutability
    • Side Effects
    • Reusability Through Higher Order Functions
    • Containers, Functors, Lists, and Streams
    • Declarative vs Imperative
    • Conclusion
  • A Functional Programmer’s Introduction to JavaScript
    • Expressions and Values
    • Types
    • Destructuring
    • Comparisons and Ternaries
    • Functions
    • Currying
    • Function Composition
    • Arrays
    • Method Chaining
    • Conclusion
  • Higher Order Functions
  • Curry and Function Composition
    • What is a curried function?
    • What is a partial application?
    • What’s the Difference?
    • What is point-free style?
    • Why do we curry?
    • Trace
    • Curry and Function Composition, Together
    • Conclusion
  • Abstraction & Composition
    • Abstraction is simplification.
    • Abstraction in Software
    • Abstraction through composition
    • How to Do More with Less Code
    • Conclusion
    • Reduce
    • Reduce is Versatile
    • Conclusion
  • Abstract Data Types and the Software Crisis
    • Common ADT Examples
    • Why ADTs?
    • History of ADTs
    • Specifications for ADTs
    • Stack ADT Example
    • Concrete Implementations
    • Conclusion
    • Glossary
  • Functors & Categories
    • Why Functors?
    • Functor Laws
    • Category Theory
    • Build Your Own Functor
    • Curried Map
    • Conclusion
  • Monads
    • You’re probably already using monads.
    • What Monads are Made of
    • Building a Kleisli Composition Function
    • The Monad Laws
    • Conclusion
  • The Forgotten History of OOP
    • The Big Idea
    • The Essence of OOP
    • What OOP Doesn’t Mean
    • What is an object?
    • We’ve lost the plot.
  • Object Composition
    • What is Object Composition?
    • Three Different Forms of Object Composition
    • Notes on Code Examples
    • Aggregation
    • Concatenation
    • Delegation
    • Conclusion
  • Factory Functions
    • Literals for One, Factories for Many
    • Returning Objects
    • Destructuring
    • Computed Property Keys
    • Default Parameters
    • Type Inference
    • Factory Functions for Mixin Composition
    • Conclusion
  • Functional Mixins
    • Motivation
    • What are mixins?
    • What is functional inheritance?
    • What is a functional mixin?
    • Composing Functional Mixins
    • When to Use Functional Mixins
    • Caveats
    • Conclusion
  • Why Composition is Harder with Classes
    • The Delegate Prototype
    • The .constructor Property
    • Class to Factory is a Breaking Change
  • Composable Custom Data Types
    • You can do this with any data type
    • Composable Currency
  • Lenses
    • Why Lenses?
    • Background
    • Lens Laws
    • Composing Lenses
  • Transducers
    • Why Transducers?
    • Background and Etymology
    • A Musical Analogy for Transducers
    • Transducers compose top-to-bottom.
    • Transducer Rules
    • Transducing
    • The Transducer Protocol
    • Conclusion
  • Elements of JavaScript Style
    • 1. Make the function the unit of composition. One job for each function.
    • 2. Omit needless code.
    • 3. Use active voice.
    • 4. Avoid a succession of loose statements.
    • 5. Keep related code together.
    • 6. Put statements and expressions in positive form.
    • 7. Use parallel code for parallel concepts.
    • Conclusion: Code should be simple, not simplistic.
  • Mocking is a Code Smell
    • TDD should lead to better design.
    • What is a code smell?
    • What is a mock?
    • What is a unit test?
    • What is test coverage?
    • What is tight coupling?
    • What causes tight coupling?
    • What does composition have to do with mocking?
    • How do we remove coupling?
    • “Code smells” are warning signs, not laws. Mocks are not evil.

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