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]>;

TypeScript Exhaustiveness Checking

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
}

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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.

TypeScript Exhaustiveness Checking Part 2

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.

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TrimRight, TrimLeft and Trim types

Split and TupleToUnion types

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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

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Creating Tuple in different ways

Creating Readonly Tuple in different ways

Creating Named Tuple in different ways

Creating Readonly Named Tuple in different ways

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Primitive Types (Boolean, Number, String)

Boolean

let isDone: boolean = false;

Number

let decimal: number = 6;
let hex: number = 0xf00d;
let binary: number = 0b1010;
let octal: number = 0o744;
let big: bigint = 100n;

String

let color: string = "blue";

let fullName: string = 'Bob Bobbington';

let age: number = 37;

let sentence: string = `Hello, my name is ${fullName}.
 
I'll be ${age + 1} years old next month.`;

Arrays

let list: number[] = [1, 2, 3];

let list2: Array<number> = [1, 2, 3];

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When you remove [keyof T] section in line 169, you will see a big difference.

As you see in the following screenshot, it created an Union Object Type, but most of the field types are never. That's why, in order to list values of type, we need to have that deleted section.