Types make big programs calm
Let's type a small shopping cart: items with quantities, adding items, and a total. Notice how the types document the code — and how mistakes can't sneak in.
Types first, then code
Typing a project like this is the everyday work of a TypeScript developer: describe the data, write honest signatures, and let the compiler point out every place the two disagree.
In typed projects, start by describing the data. Once CartItem exists, every function signature almost writes itself: adding takes a cart and returns a cart, totalling takes a cart and returns a number. The types become a plan you can check before writing any logic.
interface CartItem {
name: string;
price: number;
qty: number;
}
function addItem(cart: CartItem[], name: string, price: number): CartItem[] { … }
function removeItem(cart: CartItem[], name: string): CartItem[] { … }
function total(cart: CartItem[]): number { … }Notice the functions return a new cart instead of changing the one they receive. That style pairs beautifully with TypeScript: you can mark parameters readonly CartItem[] and the compiler will stop any accidental push or splice on the input.