TypeScript From Zero

TypeScript is the language most professional JavaScript is written in now. It's not a different language you have to relearn - it's JavaScript with a type checker bolted on top. You write code that looks almost exactly like the JS you know, add a few annotations about what your data is, and a checker catches whole categories of bugs in your editor before the code ever runs. The "undefined is not a function" and "I passed the wrong shape" mistakes stop being 2am production incidents and become red squiggles you fix as you type.

This guide takes you the whole way: from "I've never run tsc" to understanding what the type system is actually doing underneath - including the genuinely advanced features (mapped types, conditional types) that power the libraries you use. We go mental-model-first the whole way: before any annotation, you'll understand what it means and why TypeScript made the choice it did.

📝 This guide assumes you know JavaScript - values, functions, objects, async/await. If you don't yet, do JavaScript From Zero first (its final phase hands off directly to this one). TypeScript only makes sense once the JavaScript underneath does.

It's one zero-to-hero journey in two halves. Phases 1–8 are the basics - enough to type real applications confidently. Phases 9–12 are the deep half - the structural type system, utility and mapped types, conditional and template-literal types, and typing messy real-world data, the stuff that separates "writes TypeScript" from "understands the type system." Each phase carries a difficulty badge so you can see the climb.

How to read this

• New to TypeScript (but know JS)? Read 1–8 in order - each builds on the last. Type the examples yourself; watching the checker catch a mistake teaches more than reading about it. Come back for 9+ when the basics feel comfortable.
• Already using TypeScript day to day? Jump to the deep half - Phase 9: The Type System, Deep onward is where TypeScript stops being "JS with annotations" and becomes a system you can compute with.

The phases

Part 1 - The basics (🟢 Basic → 🟡 Intermediate)

1. Install & Your First Program 🟢 - tsc, a tsconfig.json, and compiling TS to JS.
2. Why Types & the Basic Types 🟢 - what the checker buys you; string/number/boolean/arrays/tuples, any vs unknown, inference.
3. Functions & Annotations 🟢 - parameter and return types, optional and default params, void.
4. Objects, Interfaces & Type Aliases 🟢 - shape types, interface vs type, optional and readonly properties.
5. Unions, Literals & Narrowing 🟡 - union types, literal types, type guards, and discriminated unions.
6. Generics 🟡 - type parameters, constraints, and writing code that works over many types safely.
7. Classes & OOP in TypeScript 🟡 - access modifiers, implements, abstract, and parameter properties.
8. Modules, tsconfig & the Build 🟡 - ES modules, strict mode, and the compiler options that actually matter.

Part 2 - Beyond the basics (🔴 Advanced) 9. The Type System, Deep 🔴 - structural typing, widening and narrowing, and how inference really works. 10. Utility & Mapped Types 🔴 - Partial/Pick/Omit/Record, keyof, and writing your own mapped types. 11. Conditional & Template Literal Types 🔴 - T extends U ? X : Y, infer, and types built from string patterns. 12. Typing the Real World 🔴 - third-party @types, declaration files, typing fetch/JSON, and taming any.

Finale 13. Where to Go Next 🟢 - React, Node, full-stack with TypeScript, and what to build.

Frameworks (React, Next, NestJS) are their own guides - they use TypeScript heavily, but this guide makes the type system itself make sense, top to bottom.

Phase 1: Install & Your First Program →