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Advanced TypeScript Exercises

If we have a type which is wrapped type like Promise. How we can get a type which is inside the wrapped type? For example if we have Promise<ExampleType> how to get ExampleType?

Take a look at below code. Write an utility type Transform which will take a generic type argument, and if it is a Promise it will evaluate to the type inside it.

type X = Promise<string>
type Y = Promise<{ field: number }>

type ResultX = Transform<X>; // ResultX type equals string
type ResultY = Transform<Y>; // ResultY type equals { field: number }

/** here your answer **/
type Transform<A> = A extends Promise<infer Inner> ? Inner : never

Understanding infer

We can use infer also with other types, consider example with Array:

type InsideArray<A> = A extends Array<infer Inside> ? Inside : never

// Str is type string
type Str = InsideArray<Array<string>>

custom parameterized types?

type Supprise<A> = { inside: A }
type UnpackSupprise<S> = S extends Supprise<infer Inside> ? Inside : never
// Num is type number
type Num = UnpackSupprise<Supprise<number>>

We can even use infer to get mapped types properties

type User = {
    id: number,
    name: string,

type Doc = {
    id: string,

type GetProperty<T, Prop extends keyof T> = T extends { [K in Prop]: infer Value } ? Value : never

type UserId = GetProperty<User, 'id'>
type DocId = GetProperty<Doc, 'id'>

Convert to tuple

type ABC<A, B, C> = { a: A, b: B, c: C }
type ABCIntoTuple<T> 
  = T extends ABC<infer A, infer B, infer C> ? [A, B, C] : never
type Example = ABC<string, boolean, number>
type ExampleTuple = ABCIntoTuple<Example>

Part 2

In this question I will ask you, why TS fails here. And I can say there is a valid reason why such construct is wrong, its not a language bug. Can you spot why, and what is example type which proves TypeScript rightly prevents such code to compile?

type User = {
  id: number;
  kind: string;

function makeCustomer<T extends User>(u: T): T {
  // Below error, why?
  return {
    kind: 'customer'

when extends only extends

At the first look object which is returned by makeCustomer is valid User type as it has both needed fields defined in the User. The crucial thing to understand is that we work here with type variable T which extends from User but it doesn't mean it is User. T is assignable to User, so it needs to have all fields which User has, but, it can have more fields!

Yes and this is exactly the issue, returned object is a User and pass all constraints of it, but doesn't pass all constraints of T which can have additional fields. We don't know what are those fields though, so in order to fix the typing we should make an object which has all fields of T, and we know all fields of T are in argument u. We can then use spread operator in order to spread all unknown fields to the newly created object.

function makeCustomer<T extends User>(u: T): T {
  // no error
    return {
    ...u, // spread all properties of u being T
    id:, // yes redundant line, leaving it for consistency
    kind: 'customer'

Part 3


Today's question is about typing function with two arguments being union type. The goal is to block possibility to pass mixed types into arguments, so if the first argument is a number then second also needs to be number, in other words there is dependency between arguments which we need to write.

function f(a: string | number, b: string | number) {
    if (typeof a === 'string') {
        return a + ':' + b; // no error but b can be number!
    } else {
        return a + b; // error as b can be number | string
f(2, 3); // correct usage
f(1, 'a'); // should be error
f('a', 2); // should be error
f('a', 'b') // correct usage


function f(T extends string | number)(a: T, b: T) {

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