Double-Char.js

/*
Given a string, you have to return a string in which each character (case-sensitive) is repeated once.

Examples (Input -> Output):

* "String"      -> "SSttrriinngg"
* "Hello World" -> "HHeelllloo  WWoorrlldd"
* "1234!_ "     -> "11223344!!__  "
Good Luck!

*/
function doubleChar(str) {
let splitString = str.split('')

return splitString.map((letter) => `${letter}${letter}`).join('')
}