Typescript/パラメーター

src/function/parameters.ts
export const isUserSignedIn = (userId:string, username?: string): boolean => {
    if(userId = "abc"){
        console.log("singin", username);
        return true;
    } else {
        console.log("not singin");
        return false;
    }
}

export const isUserSignedIn02 = (userId:string, username: string = "no name") => {
    if(userId = "abc"){
        console.log("singin", username);
        return true;
    } else {
        console.log("not singin");
        return false;
    }
}

export const sumProductsPrice = (...productsPrice/*いくつでもok*/: number[]): number => {
    return productsPrice.reduce((prevTotal/*一つ一つ取り出した一つ前の計算値*/: number, productsPrice: number) => {
        return prevTotal + productsPrice
    }, 0/*prevTotalの初期値*/)
}

type message = (message: string) => void
src/index.ts
import {isUserSignedIn, isUserSignedIn02} from "./function/parameters"

isUserSignedIn("abc", "@rise");
isUserSignedIn02("abc");
const sum = sumProductsPrice(1, 2, 3, 4, 5);
console.log(sum);