alternatingCase.js

/*
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.
*/

//Answer//

String.prototype.toAlternatingCase = function () {
let A = []
let B = this.split('')
for(let i=0; i<this.length;i++){
if (B[i]=== B[i].toLowerCase() && B[i] !== B[i].toUpperCase()) {A.push(B[i].toUpperCase())}
else {A.push(B[i].toLowerCase())}
}
return A.join("")
}