Below two code blocks are doing same job and the only difference is below second code block uses variadic tuple. Check the below links for more details.

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#more-recursive-type-aliases

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html#recursive-conditional-types

type CompareLength<Left extends any[], Right extends any[]> =
    Left['length'] extends Right['length'] ? 'equal' :
    Left extends [] ? 'shorterLeft' :
    Right extends [] ? 'shorterRight' :
    ((..._: Left) => any) extends ((_: any, ..._1: infer LeftRest) => any) ?
    ((..._: Right) => any) extends ((_: any, ..._1: infer RightRest) => any) ?
    CompareLength<LeftRest, RightRest> :
    never :
    never;

type T1 = CompareLength<[firstName: string], [lastName: string]>;

type T2 = CompareLength<[firstName: string], [lastName: string, age: number]>;

type T3 = CompareLength<[firstName: string, age: number], [lastName: string]>;

type CompareLength<Left extends any[], Right extends any[]> =
    Left['length'] extends Right['length'] ? 'equal' :
    Left extends [] ? 'shorterLeft' :
    Right extends [] ? 'shorterRight' :
    Left extends [any, ...infer L] ? 
    Right extends [any, ...infer R] ? 
    CompareLength<L, R> :
    never :
    never;

type T1 = CompareLength<[firstName: string], [lastName: string]>;

type T2 = CompareLength<[firstName: string], [lastName: string, age: number]>;

type T3 = CompareLength<[firstName: string, age: number], [lastName: string]>;

type HttpMethod = "GET" | "POST" | "PUT" | "DELETE";

const sendRequest = (url: string, method: HttpMethod) => {
    switch (method) {
        case 'DELETE':

            break;
        case 'GET':

            break;
        case 'POST':

            break;
        case 'PUT':

            break;
        default:
            const exhaustiveCheck: never = method; // ✅ no error
            throw new Error(`Unhandled case: ${exhaustiveCheck}`);
    }
};

type Fruit = 'banana' | 'orange' | 'mango';

function exhaustiveCheck(param: never): never {
    throw new Error('should not reach here')
}

function makeDessert(fruit: Fruit) {
    switch (fruit) {
        case 'banana': return 'Banana Shake'
        case 'orange': return 'Orange Juice'
    }
    exhaustiveCheck(fruit) // 🚫 ERROR! `mango` is not assignable
}

Exhaustiveness checking is a good feature when you use switch block. Assume that you need to check a value of variable which data type is union type as shown below screenshot.

Shape is a discriminated union which consists of three types named Circle, Square and Triangle.

kind field is shared in Circle, Square and Triangle types so that if developer forgets to use any of kind field value in switch block, TypeScript will error in coding-time.

Notice that there is a red line in line 347, it is due to missing triangle case in switch block.

This is the error shown in the code.

After adding triangle case in line 346, the error disappeared.

TrimRight, TrimLeft and Trim types

Split and TupleToUnion types

Function Overloading

Function Overloading with Tuple Parameters

Simple Function with Rest Parameters

Function with Rest Parameters via Tuple

Enum

Union Type

Converting Readonly Array to Union Type

Converting Array to Union Type