alternating_case.js

/*
https://www.codewars.com/kata/56efc695740d30f963000557/train/javascript

altERnaTIng cAsE <=> ALTerNAtiNG CaSe
Define String.prototype.toAlternatingCase (or a similar function/method such as to_alternating_case/toAlternatingCase/ToAlternatingCase in your selected language; see the initial solution for details) such that each lowercase letter becomes uppercase and each uppercase letter becomes lowercase. For example:

"hello world".toAlternatingCase() === "HELLO WORLD"
"HELLO WORLD".toAlternatingCase() === "hello world"
"hello WORLD".toAlternatingCase() === "HELLO world"
"HeLLo WoRLD".toAlternatingCase() === "hEllO wOrld"
"12345".toAlternatingCase() === "12345" // Non-alphabetical characters are unaffected
"1a2b3c4d5e".toAlternatingCase() === "1A2B3C4D5E"
"String.prototype.toAlternatingCase".toAlternatingCase() === "sTRING.PROTOTYPE.TOaLTERNATINGcASE"
As usual, your function/method should be pure, i.e. it should not mutate the original string.
*/

// First Attempt - Feb 2019

String.prototype.toAlternatingCase = function() {
	return this.split('')
		.map(x =>
			x == x.toUpperCase() ? x.toLowerCase() : x.toUpperCase()
		)
		.join('');
};

// Second Attempt - Feb 2020

String.prototype.toAlternatingCase = function() {
	return this.split('')
		.map(char =>
			char === char.toUpperCase()
				? char.toLowerCase()
				: char.toUpperCase()
		)
		.join('');
};