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!!__  "	
				

Solution by -

	function doubleChar(str) {
		return str.split('').map((x,i) => x + x).join('')
		}
				

Try it yourself - CodeWars

	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!!__  "	
				

Solution by -

	function doubleChar(str) {
		return str.split('').map((x,i) => x + x).join('')
		}
				

Try it yourself - CodeWars