Utility Types

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

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",
});

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>'.

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

This utility is useful for representing assignment expressions that will fail at runtime (i.e. when attempting to reassign properties of a frozen object).

Object.freeze

function freeze<Type>(obj: Type): Readonly<Type>;

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" },
};
    
cats.boris;
    
const cats: Record<CatName, CatInfo>

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

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,
};
    
todo;
    
const todo: TodoPreview
    
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
    
const todoInfo: TodoInfo = {
    title: "Pick up kids",
    description: "Kindergarten closes at 5pm",
};
    
todoInfo;
        
const todoInfo: TodoInfo

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;
}
                        

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;
}
                        

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

type T0 = string | number
type T1 = NonNullable<string[] | null | undefined>;

type T1 = string[]
                        

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>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.
        
type T6 = never
type T7 = Parameters<Function>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'. Type 'Function' provides no match for the signature '(...args: any): any'.
   
type T7 = never
                        

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>;
Type 'Function' does not satisfy the constraint 'abstract new (...args: any) => any'. Type 'Function' provides no match for the signature 'new (...args: any): any'.
type T5 = never
                        

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>;
Type 'string' does not satisfy the constraint '(...args: any) => any'.
type T7 = any
type T8 = ReturnType<Function>;
Type 'Function' does not satisfy the constraint '(...args: any) => any'. Type 'Function' provides no match for the signature '(...args: any): any'.
type T8 = any
                        

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

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

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

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 tomakeObject 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.

The ThisType<T> marker interface is simply an empty interface declared in lib.d.ts. Beyond being recognized in the contextual type of an object literal, the interface acts like any empty interface.