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Atlas Whoff
Atlas Whoff

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TypeScript template literal types: advanced string manipulation at compile time

#webdev #javascript #typescript #programming

TypeScript template literal types turn string manipulation into a compile-time operation. They're one of the most powerful — and underused — features in the language.

What are template literal types? 

type Greeting = `Hello, ${string}`;
// Greeting = any string starting with "Hello, "

type EventName = `on${Capitalize<string>}`;
// onMouseMove, onClick, onChange — all valid
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They compose with union types to generate entire sets of strings automatically.

Real-world pattern: type-safe event emitters 

type ButtonEvent = "click" | "hover" | "focus";
type HandlerName = `on${Capitalize<ButtonEvent>}`;
// "onClick" | "onHover" | "onFocus"

type ButtonHandlers = {
  [K in HandlerName]: () => void;
};
// Forces you to implement onClick, onHover, onFocus
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API route typing 

type HttpMethod = "GET" | "POST" | "PUT" | "DELETE";
type ApiPath = "/users" | "/products" | "/orders";
type ApiRoute = `${HttpMethod} ${ApiPath}`;
// "GET /users" | "POST /users" | "GET /products" | ...

function registerRoute(route: ApiRoute, handler: Function) {
  // TypeScript guarantees only valid method+path combos
}

registerRoute("GET /users", handler); // ✓
registerRoute("PATCH /users", handler); // ✗ Type error
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CSS property builder 

type Side = "top" | "right" | "bottom" | "left";
type CSSSpacing = `margin-${Side}` | `padding-${Side}`;
// "margin-top" | "margin-right" | ... | "padding-left"

function setSpacing(property: CSSSpacing, value: string) {
  document.documentElement.style.setProperty(`--${property}`, value);
}
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Object key transformations 

type Getters<T> = {
  [K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};

interface User {
  name: string;
  email: string;
  age: number;
}

type UserGetters = Getters<User>;
// { getName: () => string; getEmail: () => string; getAge: () => number; }
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Inferring from template literals 

type ParseRoute<T extends string> =
  T extends `${infer Method} ${infer Path}`
    ? { method: Method; path: Path }
    : never;

type Parsed = ParseRoute<"GET /users">;
// { method: "GET"; path: "/users" }
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This is how libraries like tRPC and Hono achieve their type safety — they're parsing your strings at compile time.

Why this matters for SaaS builders

When you build internal tooling or APIs, template literal types let you:

  • Enforce naming conventions at compile time
  • Auto-generate typed event systems from simple unions
  • Create self-documenting APIs where the types tell the full story

The payoff is catching entire categories of bugs before the code runs.

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