TypeScript Essentials Cheat Sheet

type Draft = Partial<User>;

Required<T> goes the other way.

type Preview = Pick<User, "id" | "name">;
type Safe = Omit<User, "password">;

const byId: Record<string, User> = {};

Reading byId["missing"] types as User unless noUncheckedIndexedAccess is on.

type Result = ReturnType<typeof fetchUser>;

For an async function wrap it: Awaited<ReturnType<typeof fetchUser>>.

function isCat(pet: Pet): pet is Cat {
  return "meow" in pet;
}

After if (isCat(pet)) the compiler treats pet as Cat in that branch.

switch (shape.kind) {
  case "circle":
    return area(shape);
  default: {
    const unreachable: never = shape;
    return unreachable;
  }
}

Add a kind to the union and the assignment to never becomes an error until the switch handles it.

const ROLES = ["user", "admin"] as const;
type Role = (typeof ROLES)[number];

Without as const the array is string[] and Role is just string.

type ConfigKey = keyof typeof config;

const config = { port: 3030, host: "localhost" } satisfies Config;

config.port stays the literal 3030. With const config: Config = ... it widens to number.

function longest<T extends { length: number }>(a: T, b: T): T {
  return a.length >= b.length ? a : b;
}

Works for strings, arrays, and anything else with a length.

npx tsc --noEmit

Point it at another config with -p tsconfig.scripts.json.

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true
  }
}

With the second flag, indexing an array or a record gives T | undefined, which is what it is at runtime.