This article is meant for people who know the basics of TypeScript but haven’t used it in a large project, and/or haven’t explored its advanced types.
My goal is to convince you, based on a ton of examples, that TypeScript’s powerful advanced type system can provide satisfying solutions for complex web engineering problems and can be used for mind-blowing and fun modeling.
Watch this video and refer to code snippets below.
Building Blocks
These are types you can find on the official TypeScript documentation. They are the building blocks for more advanced/fancy stuff. Think of this section as a cheat-sheet.
Intersection types
Get all members from multiple types:
javascripttype Duck = Quackable & Swimmable;
If you think of types as sets, what you get here is the intersection of two types, which is a smaller set. This can be confusing as the actual resulting type has a “larger” interface, i.e. the union of the members of both types.
Union types
Get common members from multiple types (the resulting type is the union between the two, i.e. a larger set):
javascripttype Flyable = Eagle | Butterfly;
Custom Type Guards
Conditional type cast:
javascriptconst canFly = (animal: Animal): animal is Flyable =>typeof (animal as any).fly === 'function';if (canFly(animal)) {animal.fly();}
With the is keyword you can define functions to determine “by hand” whether a value belongs to a type, and TypeScript then assumes that type in the true part of a conditional statement.
Type Assertion
A cast shorthand if you know that a specific variable is definitely defined:
javascriptperson.children[0]!.name;
Literal Types
You can use actual primitive literals (strings, booleans etc.) as types:
javascripttype Move = 'ROCK' | 'PAPER' | 'SCISSOR';
If you think of types as sets, the type 'ROCK' is the set containing only the value 'ROCK'.
Never
An expression that never resolves:
javascriptconst throws = (): never => {throw new Error('This never returns');}const loops = (): never => {while (true) {console.log('This never returns either');}}
This type will magically appear in many surprising spots in the examples below.
Unknown
A safer type than any for variables that you can’t trust:
javascriptlet a: string;let x: any;a = x; // Compilesx = a; // Compileslet y: unknown;y = a; // Compilesa = y; // Does not compile
Index Types
Access the types of object keys or values:
javascripttype Duck = {colors: string;feathers: number;}type DuckProps = keyof Duck; // = 'colors' | 'feathers'type ColorType = Duck['colors']; // = stringtype DuckValues = Duck[DuckProps] // = string | number
Mapped Types
Derive an object type from another:
javascripttype Map1 = { [key: string]: string };type Map2 = { [P in 'a' | 'b' | 'c']: string };// = { a: string, b: string, c: string }type Map3 = { [P in 'a' | 'b' | 'c']: P };// = { a: 'a', b: 'b', c: 'c' }
This powerful abstraction will make more sense when you see specific applications of it.
Conditional Types
Let a type depend on another type with a condition:
javascripttype StringOrNumber<T> = T extends boolean ? string : number;type T1 = StringOrNumber<true>; // stringtype T2 = StringOrNumber<false>; // stringtype T3 = StringOrNumber<Object>; // number
Here’s an actual application for conditional types:
javascripttype TypeName<T> =T extends string ? "string":T extends number ? "number" :T extends boolean ? "boolean" :T extends undefined ? "undefined" :T extends Function ? "function" :"object";type T0 = TypeName<string>; // "string"
With the infer keyword you can introduce a type variable, the type of which will be inferred:
javascripttype ElementType<T> = T extends (infer U)[] ? U : never;type T = ElementType<[]>; // = nevertype T1 = ElementType<string[]>; // = string
Misc Patterns
Functional Programming
This is not exactly related to advanced types, rather it’s an illustration of how one can program in a lightweight but type safe way with TypeScript. In Java world, in the Clean Code paradigm there are two main categories of classes: objects and data types. Data types represent data (think getters and setters), while objects represent functionality (think methods). With TypeScript one can simplify this approach even more by just using interfaces to represent data and functions to manipulate data. No bulky boilerplate, no constructors, getters and setters needed. Just return a plain object and typescript will ensure it is correctly structured.
javascriptinterface RawPerson {identifier: number;first_name: string;last_name: string;}interface Person {id: string;fullName: string;}const transformPerson = (raw: RawPerson): Person => {return {id: `${raw.identifier}`,fullName: `${raw.first_name} ${raw.last_name}`,}}
Discriminate Unions
TypeScript understands you have exhaustively checked all possible variants of a type in a switch statement, and won’t force you to use a default statement.
javascripttype Eagle = {kind: 'eagle';fly: () => 'fly';};type Duck = {kind: 'duck';quack: () => 'quack';};type Bird = {kind: 'bird';};type Animal = Eagle | Duck | Bird;const doSomething = (animal: Animal): string => {switch (animal.kind) {case 'eagle':return animal.fly();case 'duck':return animal.quack();case 'bird':return "animal.quack()";}}
Derive Types From Constants
Often we need literal types both as values and as types. To avoid redundancy derive the latter from the former.
javascriptconst MOVES = {ROCK: { beats: 'SCISSOR' },PAPER: { beats: 'ROCK' },SCISSOR: { beats: 'PAPER' },};type Move = keyof typeof MOVES;const move: Move = 'ROCK';
Untrusted User Input
Untrusted user input is an application for unknown.
javascriptconst validateInt = (s: unknown): number => {let n;switch (typeof s) {case 'number':// handlecase 'string':// handle// other casesdefault:throw new Error('Not a number.');}}
Mapped Types
Create a readonly version of a type:
javascripttype Readonly<T> = { readonly [P in keyof T]: T[P] };type ReadonlyDuck = Readonly<Duck>;// = { readonly color: string; readonly feathers: number }
Derive a partial version of a type:
javascripttype Partial<T> = { [P in keyof T]?: T[P] };type PartialDuck = Partial<Duck>;// = { color?: string; feathers?: number }
Derive a version of a type where all fields are required:
javascripttype PartialDuck = {color?: string;feathers?: number;}type Required<T> = { [P in keyof T]-?: T[P] };type Duck = Required<PartialDuck>;// = { color: string; feathers: number }
Derive a nullable version of a type:
javascripttype Nullable<T> = { [P in keyof T]: T[P] | null }type NullableDuck = Partial<Duck>;// = { color: string | null; feathers: number | null }
Derive a type with only specific fields (Pick):
javascripttype Pick<T, K extends keyof T> = { [P in K]: T[P] };type ColorDuck = Pick<Duck, 'color'>;
An example from practice, where we select specific columns from a SQL table, and TypeScript automatically derives the partial type for us:
javascriptasync function fetchPersonById<T extends keyof Person>(id: string,...fields: T[]): Promise<Pick<Reaction, T>> {return await knex('Person').where({ id }).select(fields).first();}const reaction = await fetchPersonById(id, 'name', 'age');// = { name: string, age: number }
Derive a record (all properties have same given type) from a type:
javascripttype Record<K extends string, T> = { [P in K]: T };type Day = 'Monday' | 'Tuesday' | 'Wednesday' | 'Thursday' | 'Friday' | 'Saturday' | 'Sunday';type EverydayParty = Record<Day, 'party'>;/*= {Monday: 'party';Tuesday: 'party';Wednesday: 'party';Thursday: 'party';Friday: 'party';Saturday: 'party';Sunday: 'party';}*/
Conditional Types
Filter union types
Filter / extract a subset from a type:
javascripttype Filter<T, U> = T extends U ? T : never;type T1 = Filter<"a" | "b", "a">; // = "a"// Natively available as Extracttype T2 = Extract<"a" | "b", "a"> // = "a"
Diff / exclude a subset from a type:
javascripttype Diff<T, U> = T extends U ? never : T;type T1 = Diff<"a" | "b", "a">; // = "b"// Natively available as Excludetype T2 = Exclude<"a" | "b", "a"> // = "b"
Exclude null and undefined from a typed (thus making it non-nullable):
javascripttype NonNullable<T> = Diff<T, null | undefined>;type T = NonNullable<string | null | undefined>; // = string
Omit specific fields from a type:
javascripttype Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;type PartialDuck = Omit<Duck, 'feathers'>;// = { color: string }
Here is an example from practice (how to type a React.js HOC correctly):
javascriptexport interface WithUserProps {user: User;}export const withUser = <P extends WithUserProps>(Component: React.ComponentType<P>) => (props: Omit<P, WithUserProps>) => (<Component {...props} user={getUser()} />)const UsernameComponent = ({ user, message }: { user: User, message: string }) => (<div>Hi {user.username}! {message}</div>)const Username = withUser(UsernameComponent); // ({ message }) => JSX.Element
Combining with mapped types
Get the function properties of an object:
javascripttype FunctionPropertyNames<T> = {[K in keyof T]: T[K] extends Function ? K : never}[keyof T];type FunctionProperties<T> = Pick<T, FunctionPropertyNames<T>>;type Duck = {color: string,fly: () => void,}type T1 = FunctionProperties<Duck>; // = { fly: () => void }type T2 = FunctionPropertyNames<Duck>; // = "fly"
Get the non-function properties of an object:
javascripttype NonFunctionPropertyNames<T> = {[K in keyof T]: T[K] extends Function ? never : K}[keyof T];type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;type Duck = {color: string,fly: () => void,}type T1 = NonFunctionProperties<Duck>; // = { color: string }type T2 = NonFunctionPropertyNames<Duck>; // = "color"
Get the return type of a function:
javascripttype ReturnType<T> = T extends (...args: any[]) => infer R ? R : any;const x = (a: string): number => 1;type R = ReturnType<typeof x>; // = number
Get the instance type of a constructor function:
javascripttype InstanceType<T extends new (...args: any[]) => any>= T extends new (...args: any[]) => infer R ? R : any;class Duck {}const make2 = <T extends new (...args: any[]) => any>(constructor: T): [InstanceType<T>, InstanceType<T>] =>[new constructor(), new constructor()]const ducks = make2(Duck); // = [Duck, Duck]
Promisifying Redis
Another example from practice. All methods in the npm redis library use callbacks. We like to use promises and async/await. We can derive the new type from the old based on how we build it.
javascriptimport { ClientOpts, createClient } from 'redis';import { promisify } from 'util';export const promisifyRedis = (opts: ClientOpts) => {const redis = createClient(opts);const promisifiedRedis = {setex: promisify(redis.setex),get: promisify(redis.get),};const wrappedRedis: typeof promisifiedRedis = {} as any;for (const key of Object.keys(promisifiedRedis) as (keyof typeof promisifiedRedis)[]) {wrappedRedis[key] = promisifiedRedis[key].bind(redis);}return wrappedRedis;};export type Redis = ReturnType<typeof promisifyRedis>;/* =type Redis = {setex: (arg1: string, arg2: number, arg3: string) => Promise<string>;get: (arg1: string) => Promise<...>;}*/
Conclusion
I hope to have convinced you of the power of TypeScript. Now go ahead and TYPE!