Functional Programming with TypeScript - Expanded Project Guides

Generated from: FUNCTIONAL_PROGRAMMING_TYPESCRIPT_LEARNING_PROJECTS.md

This folder contains deep-dive guides for each project in the learning path.

Overview

Master functional programming principles through TypeScript—transforming how you think about code from imperative commands to pure, composable transformations. Learn to eliminate null pointer exceptions, make illegal states unrepresentable, and build complex systems from simple, testable functions that compose like LEGO blocks.

After completing this journey, you’ll understand why companies like Facebook (React), Netflix (RxJS), and Elm use functional patterns. You’ll stop writing for loops and if (x !== null) checks by default. Instead, you’ll see data transformations as pipelines, errors as values, and side effects as explicitly managed monads.

Project Index

#	Project	Difficulty	Time	Key Focus
1	Immutable Todo List	Beginner	Weekend	Immutability, Pure Functions, Data Transformations
2	The “Pipe” Text Processor	Beginner	Weekend	Function Composition, Pipelines, Point-Free Style
3	The “Maybe” Null Handler	Intermediate	1 Week	Functors, Monads, Optional Values
4	The “Either” Validation Library	Intermediate	1-2 Weeks	Error Handling, Bifunctors, Applicatives
5	Functional JSON Parser	Expert	2-3 Weeks	Parser Combinators, Recursion, Grammar
6	Lazy Stream Library	Advanced	2 Weeks	Lazy Evaluation, Infinite Data Structures, Fusion
7	The “IO” Effect Handler	Advanced	1 Week	Side Effects, Referential Transparency, IO Monad
8	Functional Reactive UI	Master	2-3 Weeks	Observables, Event Streams, State Management

Learning Paths

For Beginners

Start with projects #1 and #2. These will rewire your brain from imperative loops to data transformations.

Project 1 (Immutable Todo) → Project 2 (Pipe Processor)
         ↓
      Project 3 (Maybe Monad)

For Intermediate Developers

Jump to projects #3 and #4. This is the biggest ROI—you’ll immediately start writing safer code.

Project 3 (Maybe) → Project 4 (Either) → Project 7 (IO)

For Advanced Developers

Focus on projects #5, #6, and #8. These will make you a functional programming wizard.

Project 5 (Parser) → Project 6 (Lazy Streams) → Project 8 (Reactive UI)

Core Concepts Covered

Concept	Projects
Immutability	1, 2, 3, 4, 5, 6, 7, 8
Pure Functions	1, 2, 3, 4, 5, 6, 7
Function Composition	2, 3, 4, 5, 6, 8
Functors	3, 4, 5, 6
Monads	3, 4, 5, 7
Applicative Functors	4, 5
Parser Combinators	5
Lazy Evaluation	6
Reactive Programming	8
Algebraic Data Types	3, 4, 5

Prerequisites

Before starting this journey, you should have:

TypeScript fundamentals: Classes, interfaces, generics, union types
JavaScript array methods: map, filter, reduce, flatMap
Basic programming experience: Variables, functions, loops, conditionals
Development environment: Node.js, npm, VSCode or similar editor

Capstone Project

After completing projects 1-8, you’ll be ready for the Declarative Spreadsheet Engine—a spreadsheet application running in the browser that integrates all concepts:

Immutability: Every cell update produces a new grid state (Undo/Redo)
Parser Combinators: Parse formulas like =SUM(A1:A5) + B2
Reactivity: Cells are Observables that auto-update dependencies
Error Handling: Formula errors propagate without crashing
Lazy Evaluation: Support infinite ranges and huge datasets

Expected Outcomes

After completing these projects, you will:

Stop using for loops and mutation by default
See null and undefined as problems solved with Types, not Checks
Understand how to build complex systems by composing small, pure functions
Be able to implement Redux, RxJS, or Parser logic from scratch
Have 8 working projects demonstrating deep FP understanding

Resources

Essential Books

Professor Frisby’s Mostly Adequate Guide to FP (free online)
Hands-On Functional Programming with TypeScript by Remo H. Jansen
Programming TypeScript by Boris Cherny
Domain Modeling Made Functional by Scott Wlaschin

Libraries to Explore

fp-ts: Typed functional programming in TypeScript
RxJS: Reactive Extensions for JavaScript
io-ts: Runtime type validation
Effect-TS: Production-grade effect system

Start your journey with Project 1: Immutable Todo List