Utility Types

Question 1

Awaited<Type>

Answer 1

This type is meant to model operations like await in async functions, or the .then() method on Promises - specifically, the way that they recursively unwrap Promises.

Example

type A = Awaited<Promise<string>>;
// type A = string
type B = Awaited<Promise<Promise<number>>>;
// type B = number
type C = Awaited<boolean | Promise<number>>;
// type C = number | boolean

“Awaited<Type>” (typescriptlang.org). Retrieved June 6, 2023.

Question 2

Partial<Type>

Answer 2

Constructs a type with all properties of Type set to optional. This utility will return a type that represents all subsets of a given type.

Example:

interface Todo {
  title: string;
  description: string;
}
 
function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
  return { ...todo, ...fieldsToUpdate };
}
 
const todo1 = {
  title: "organize desk",
  description: "clear clutter",
};
 
const todo2 = updateTodo(todo1, {
  description: "throw out trash",
});

“Partial<Type>” (typescriptlang.org). Retrieved June 6, 2023.

Question 3

Required<Type>

Answer 3

Constructs a type consisting of all properties of Type set to required. The opposite of Partial.

Example

interface Props {
  a?: number;
  b?: string;
}
 
const obj: Props = { a: 5 };
 
const obj2: Required<Props> = { a: 5 };
// Property 'b' is missing in type '{ a: number; }' but required in type 'Required<Props>'.

“Required<Type>” (typescriptlang.org). Retrieved June 6, 2023.

Question 4

Readonly<Type>

Answer 4

Constructs a type with all properties of Type set to readonly, meaning the properties of the constructed type cannot be reassigned.

Example

interface Todo {
  title: string;
}
 
const todo: Readonly<Todo> = {
  title: "Delete inactive users",
};
 
todo.title = "Hello";
// Error: Cannot assign to 'title' because it is a read-only property.

“Readonly<Type>’” (typescriptlang.org). Retrieved June 6, 2023.

Question 5

Record<Keys, Type>

Answer 5

Constructs an object type whose property keys are Keys and whose property values are Type. This utility can be used to map the properties of a type to another type.

Example

interface CatInfo {
  age: number;
  breed: string;
}
 
type CatName = "miffy" | "boris" | "mordred";
 
const cats: Record<CatName, CatInfo> = {
  miffy: { age: 10, breed: "Persian" },
  boris: { age: 5, breed: "Maine Coon" },
  mordred: { age: 16, breed: "British Shorthair" },
};

“Record<Keys, Type>” (typescriptlang.org). Retrieved June 6, 2023.

Question 6

Pick<Type, keys>

Answer 6

Constructs a type by picking the set of properties Keys (string literal or union of string literals) from Type.

Example

interface Todo {
  title: string;
  description: string;
  completed: boolean;
}
 
type TodoPreview = Pick<Todo, "title" | "completed">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
};

“Pick<Type, Keys>” (typescriptlang.org). Retrieved June 6, 2023.

Question 7

Omit<Type, Keys>

Answer 7

Constructs a type by picking all properties from Type and then removing Keys (string literal or union of string literals). The opposite of Pick.

Example

interface Todo {
  title: string;
  description: string;
  completed: boolean;
  createdAt: number;
}
 
type TodoPreview = Omit<Todo, "description">;
 
const todo: TodoPreview = {
  title: "Clean room",
  completed: false,
  createdAt: 1615544252770,
};
 
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
 
const todoInfo: TodoInfo = {
  title: "Pick up kids",
  description: "Kindergarten closes at 5pm",
};

“Omit<Type, Keys>” (typescriptlang.org). Retrieved June 6, 2023.

Question 8

Exclude<UnionType, ExcludedMembers>

Answer 8

Constructs a type by excluding from UnionType all union members that are assignable to ExcludedMembers.

Example

type T0 = Exclude<"a" | "b" | "c", "a">;
// type T0 = "b" | "c"

type T1 = Exclude<"a" | "b" | "c", "a" | "b">;
// type T1 = "c"

type T2 = Exclude<string | number | (() => void), Function>;
// type T2 = string | number
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T3 = Exclude<Shape, { kind: "circle" }>
/*
    type T3 = {
        kind: "square";
        x: number;
    } | {
        kind: "triangle";
        x: number;
        y: number;
    }
*/

“Exclude<UnionType, ExcludedMembers>” (typescriptlang.org). Retrieved June 7, 2023.

Question 9

Extract<Type, Union>

Answer 9

Constructs a type by extracting from Type all union members that are assignable to Union.

Example

type T0 = Extract<"a" | "b" | "c", "a" | "f">;
// type T0 = "a"

type T1 = Extract<string | number | (() => void), Function>;
// type T1 = () => void
 
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; x: number }
  | { kind: "triangle"; x: number; y: number };
 
type T2 = Extract<Shape, { kind: "circle" }>
/* 
    type T2 = {
        kind: "circle";
        radius: number;
    }
*/

“Extract<Type, Union>” (typescriptlang.org). Retrieved June 7, 2023.

Question 10

NonNullable<Type>

Answer 10

Constructs a type by excluding null and undefined from Type.

Example

type T0 = NonNullable<string | number | undefined>;
// type T0 = string | number

type T1 = NonNullable<string[] | null | undefined>;
// type T1 = string[]

“NonNullable<Type>” (typescriptlang.org). Retrieved June 7, 2023.

Question 11

Parameters<Type>

Answer 11

Constructs a tuple type from the types used in the parameters of a function type Type.

Example

declare function f1(arg: { a: number; b: string }): void;
 
type T0 = Parameters<() => string>;
// type T0 = []

type T1 = Parameters<(s: string) => void>;
// type T1 = [s: string]

type T2 = Parameters<<T>(arg: T) => T>;
// type T2 = [arg: unknown]

type T3 = Parameters<typeof f1>;
/*     
    type T3 = [arg: {
        a: number;
        b: string;
    }]
*/

type T4 = Parameters<any>;
// type T4 = unknown[]

type T5 = Parameters<never>;
// type T5 = never

type T6 = Parameters<string>;
// Error: Type 'string' does not satisfy the constraint '(...args: any) => any'.
     
type T7 = Parameters<Function>;
// Error: Type 'Function' does not satisfy the constraint '(...args: any) => any'.
 // Error: Type 'Function' provides no match for the signature '(...args: any): any'.

“Parameters<Type>” (typescriptlang.org). Retrieved June 7, 2023.

Question 12

ConstructorParameters<Type>

Answer 12

Constructs a tuple or array type from the types of a constructor function type. It produces a tuple type with all the parameter types (or the type never if Type is not a function).

Example

type T0 = ConstructorParameters<ErrorConstructor>; 
// type T0 = [message?: string]

type T1 = ConstructorParameters<FunctionConstructor>; 
// type T1 = string[]

type T2 = ConstructorParameters<RegExpConstructor>; 
// type T2 = [pattern: string | RegExp, flags?: string]

class C {
  constructor(a: number, b: string) {}
}

type T3 = ConstructorParameters<typeof C>; 
// type T3 = [a: number, b: string]

type T4 = ConstructorParameters<any>; 
// type T4 = unknown[]
 
type T5 = ConstructorParameters<Function>;
// Error: Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
// Error: Type 'Function' provides no match for the signature 'new (...args: any): any'.

“ConstructorParameters<Type>” (typescriptlang.org). Retrieved June 7, 2023.

Question 13

ReturnType<Type>

Answer 13

Constructs a type consisting of the return type of function Type.

Example

declare function f1(): { a: number; b: string };
 
type T0 = ReturnType<() => string>;
// type T0 = string

type T1 = ReturnType<(s: string) => void>;
// type T1 = void

type T2 = ReturnType<<T>() => T>;
// type T2 = unknown

type T3 = ReturnType<<T extends U, U extends number[]>() => T>;
// type T3 = number[]

type T4 = ReturnType<typeof f1>;
/* 
    type T4 = {
        a: number;
        b: string;
    }
*/
		
type T5 = ReturnType<any>;
// type T5 = any

type T6 = ReturnType<never>;
// type T6 = never

type T7 = ReturnType<string>;
// Error" Type 'string' does not satisfy the constraint '(...args: any) => any'.

type T8 = ReturnType<Function>;
// Error: Type 'Function' does not satisfy the constraint '(...args: any) => any'.
// Errror: Type 'Function' provides no match for the signature '(...args: any): any'.

“ReturnType<Type>” (typescriptlang.org). Retrieved June 7, 2023.

Question 14

InstanceType<Type>

Answer 14

Constructs a type consisting of the instance type of a constructor function in Type.

Example

class C {
  x = 0;
  y = 0;
}
 
type T0 = InstanceType<typeof C>;
// type T0 = C

type T1 = InstanceType<any>;
// type T1 = any

type T2 = InstanceType<never>;
// type T2 = never

type T3 = InstanceType<string>;
// Error: Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.
     
type T4 = InstanceType<Function>;
// Error: Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'.
// Error: Type 'Function' provides no match for the signature 'new (...args: any): any'.

“InstanceType<Type>” (typescriptlang.org). Retrieved June 9, 2023.

Question 15

ThisParameterType<Type>

Answer 15

Extracts the type of the this parameter for a function type, or unknown if the function type has no this parameter.

Example:

function toHex(this: Number) {
  return this.toString(16);
}
 
function numberToString(n: ThisParameterType<typeof toHex>) {
  return toHex.apply(n);
}

“ThisParameterType<Type>” (typescriptlang.org). Retrieved June 9, 2023.

Question 16

OmitThisParameter<Type>

Answer 16

Removes the this parameter from Type. If Type has no explicitly declared this parameter, the result is simply Type. Otherwise, a new function type with no this parameter is created from Type. Generics are erased and only the last overload signature is propagated into the new function type.

Example

function toHex(this: Number) {
  return this.toString(16);
}
 
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);
 
console.log(fiveToHex());

“OmitThisParameter<Type>” (typescriptlang.org). Retrieved June 9, 2023.

Question 17

ThisType<Type>

Answer 17

This utility does not return a transformed type. Instead, it serves as a marker for a contextual this type. Note that the noImplicitThis flag must be enabled to use this utility.

Example

type ObjectDescriptor<D, M> = {
  data?: D;
  methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M
};
 
function makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M {
  let data: object = desc.data || {};
  let methods: object = desc.methods || {};
  return { ...data, ...methods } as D & M;
}
 
let obj = makeObject({
  data: { x: 0, y: 0 },
  methods: {
    moveBy(dx: number, dy: number) {
      this.x += dx; // Strongly typed this
      this.y += dy; // Strongly typed this
    },
  },
});
 
obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);

In the example above, the methods object in the argument to makeObject has a contextual type that includes ThisType<D & M> and therefore the type of this in methods within the methods object is { x: number, y: number } & { moveBy(dx: number, dy: number): void }. Notice how the type of the methods property simultaneously is an inference target and a source for the this type in methods.

“ThisType<Type>” (typescriptlang.org). Retrieved June 9, 2023.

Question 18

List all Intrinsic string manipulation Types

Answer 18

• Uppercase<StringType> - Converts each character in the string to the uppercase version.
• Lowercase<StringType> - Converts each character in the string to the lowercase equivalent.
• Capitalize<StringType> - Converts the first character in the string to an uppercase equivalent.
• Uncapitalize<StringType> - Converts the first character in the string to a lowercase equivalent.

Example:

type Greeting = "Hello, world"
type ShoutyGreeting = Uppercase<Greeting> 
// type ShoutyGreeting = "HELLO, WORLD"
type QuietGreeting = Lowercase<Greeting> 
// type QuietGreeting = "hello, world"
type Greeting = Capitalize<QuietGreeting>; 
// type Greeting = "Hello, world"
type UncomfortableGreeting = Uncapitalize<ShoutyGreeting>; 
// type UncomfortableGreeting = "hELLO WORLD"

“Intrinsic String Manipulation Types” (typescriptlang.org). Retrieved April 13, 2023.