Most backend code does not become hard to change because of one bad decision. It drifts: an OrderService picks up one more responsibility, a switch grows one more case, a payment SDK leaks into a controller. Design principles and patterns are the names for the refactors that stop that drift, and system design is what the same thinking looks like once the boxes are services instead of classes.

This guide goes from the smallest scale to the largest. It starts with how to study patterns at all, moves through SOLID and ten Gang of Four patterns, and ends with two classic system design problems and a framework for the system design interview.

Who this is for

You write backend or full-stack JavaScript and TypeScript and you want to go from "it works" to "it is easy to change", or you are preparing for a senior interview where "which pattern would you use here?" and "design a news feed" are both on the table. You should be comfortable with classes, interfaces, async/await and basic Node.js.

How each chapter works

Every pattern chapter follows the same shape:

  1. A piece of code with a real smell, and why it hurts.
  2. The pattern, applied step by step to that code, in TypeScript.
  3. How the pattern looks in modern JavaScript, where a function or a module often replaces a class.
  4. When to use it, when not to, and the usual pitfalls.
  5. Exercises.

All examples run on Node.js 22+ with tsx or Bun. Copy a block into a file and run it; the output is shown in comments.

The learning path

About 4 to 6 hours a week gives you a comfortable five-week plan.

Week 1: foundations

Week 2: creational patterns

Week 3: structural patterns

Week 4: behavioral patterns

Week 5: from patterns to system design

A word of caution

Patterns are tools, not goals. The best outcome of this guide is not that you use all ten, it is that you can look at a piece of code, name what is wrong with it, and pick the smallest change that fixes it. Sometimes that change is a pattern. Often it is just splitting a function.