TypeScript Cheat Sheet

TypeScript type system reference — basic types, generics, utility types, type guards, and declaration patterns.

Basic Types

Stringlet s: string = 'hello'

Numberlet n: number = 42

Booleanlet b: boolean = true

Arraylet a: number[] = [1,2,3]

Tuplelet t: [string, number] = ['a', 1]

Anylet x: any = 'anything'

Unknownlet u: unknown = getData()

Neverfunction fail(): never { throw Error() }

Voidfunction log(): void { console.log('hi') }

Interfaces & Types

Interfaceinterface User { name: string; age: number }

Type aliastype ID = string | number

Optionalinterface Cfg { debug?: boolean }

Readonlyinterface Pt { readonly x: number }

Extendinterface Admin extends User { role: string }

Intersectiontype AdminUser = User & { role: string }

Generics

Functionfunction id<T>(x: T): T { return x }

Interfaceinterface Box<T> { value: T }

Constraintfunction len<T extends { length: number }>(x: T)

Defaultinterface Api<T = unknown> { data: T }

Utility Types

Partial<T>Partial<User>All properties optional

Required<T>Required<User>All properties required

Readonly<T>Readonly<User>All properties readonly

Pick<T,K>Pick<User, 'name' | 'age'>Select properties

Omit<T,K>Omit<User, 'password'>Exclude properties

Record<K,V>Record<string, number>Key-value map type

ReturnType<T>ReturnType<typeof fn>Function return type

Parameters<T>Parameters<typeof fn>Function param types

Type Guards

typeofif (typeof x === 'string') { ... }

instanceofif (x instanceof Date) { ... }

inif ('name' in obj) { ... }

Custom guardfunction isUser(x: any): x is User { ... }

Assertionfunction assert(x: any): asserts x is User { }

Enums & Literals

Enumenum Dir { Up, Down, Left, Right }

String enumenum Color { Red = 'RED', Blue = 'BLUE' }

Const enumconst enum Status { Active, Inactive }

Literaltype Theme = 'light' | 'dark'

Template literaltype Event = `on${string}`

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