[Typescript Use Case] Typescript use case collection

typescript_dynamic-key-function.ts

/**
* A function returns a object, the key of the object is the value of the input object
* Declare the type for the following fuction
* e.g. const mappingFunc = obj => ({[obj.name]: true})
**/

const mappingFunc = <T extends string>(arg: {name: T}): Record<T, boolean> => ({
  [arg.name]: true
})

const obj = {
  name: 'foo' as 'foo' // <-- narrow down the type from `string` to `"foo"` (String Literal Type)
}

const output = mappingFunc<typeof obj.name>(obj);
console.log(output.foo) // true
console.log(output.bar) // error:  TS2339: Property 'bar' does not exist on type 'Record<"foo", boolean>'.

Variadic_Tuple_Types.ts

/* Variadic Tuple Types (generic tuple with spread) */

function tail<T extends any[]>(arr: readonly [any, ...T]) {
    const [_ignored, ...rest] = arr;
    return rest;
}

const myTuple = [1, 2, 3, 4] as const;
const myArray = ["hello", "world"];

// type [2, 3, 4]
const r1 = tail(myTuple);

// type [2, 3, ...string[]]
const r2 = tail([...myTuple, ...myArray] as const);


//  well-typed signature for concat:
type Arr = readonly any[];

function concat<T extends Arr, U extends Arr>(arr1: T, arr2: U): [...T, ...U] {
    return [...arr1, ...arr2];
}