TypeScript Type Guards Cheat Sheet

function format(value: string | number) {  if (typeof value === 'string') {    return value.toUpperCase(); // narrowed to string  }  return value.toFixed(2); // narrowed to number}class Dog { bark() {} }class Cat { meow() {} }function speak(animal: Dog | Cat) {  if (animal instanceof Dog) {    animal.bark(); // narrowed to Dog  } else {    animal.meow(); // narrowed to Cat  }}

interface Fish { swim(): void; }interface Bird { fly(): void; }function isFish(pet: Fish | Bird): pet is Fish { // type predicate  return (pet as Fish).swim !== undefined;}function move(pet: Fish | Bird) {  if (isFish(pet)) {    pet.swim(); // narrowed to Fish  } else {    pet.fly(); // narrowed to Bird  }}

interface Admin { role: 'admin'; permissions: string[]; }interface Member { role: 'member'; }function describe(user: Admin | Member) {  if ('permissions' in user) { // 'in' narrows by property presence    console.log(user.permissions);  }  if (user.role === 'admin') { // discriminant narrowing    console.log(user.permissions);  }}

function assertIsString(val: unknown): asserts val is string {  if (typeof val !== 'string') {    throw new Error('Expected a string');  }}function process(input: unknown) {  assertIsString(input);  input.toUpperCase(); // narrowed to string after the assertion call}function assertDefined<T>(val: T): asserts val is NonNullable<T> {  if (val === null || val === undefined) throw new Error('Value is required');}