Striktheidsopties

// tsconfig.json (excerpt)
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true
  }
}

// Effect demo
function sum(a, b) { // Error: implicit any
  return a + b;
}

function sumOk(a: number, b: number): number {
  return a + b;
}

// exactOptionalPropertyTypes: true
interface User { name?: string }

const u1: User = {};          // ok: name is absent
const u2: User = { name: undefined }; // Error when exactOptionalPropertyTypes is true

function greet(u: User) {
  if ("name" in u) {
    // name exists but could be undefined without this flag
    console.log("Hi", u.name);
  }
}

// noUncheckedIndexedAccess: true
const xs: number[] = [1,2,3];
const maybe = xs[5]; // type: number | undefined
if (maybe !== undefined) {
  console.log(maybe.toFixed(0));
}

// Also affects records/maps
const byId: Record<string, { id: string }> = {};
const item = byId["x"]; // { id: string } | undefined
if (item) {
  console.log(item.id);
}

// tsconfig.json (excerpt)
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "exactOptionalPropertyTypes": true,
    "noUncheckedIndexedAccess": true
  }
}