Book: TypeScript Essentials Also by me: The TypeScript Library — the 5-book collection My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub You open a file in the orders service you have not touched in two years. There is a function called monthlyRevenueByCategory . It is thirty-two lines. The first eight declare an empty object, an empty array, and three counters. The next twenty are nested if blocks, an early continue , and a // special case comment that does not explain itself. The last four flatten the result. You have to add one thing (group by region as well as by category) and you spend forty minutes reading the loop before you trust yourself to touch it. That function is the cost of writing TypeScript like it was 2014. The runtime caught up. Array methods, Object.groupBy , generators, and the ES2025 iterator helpers cover roughly nine out of every ten places you would have reached for a for loop in 2018.…