Was trying to refactor code and not sure where I am going wrong?

Original code i wrote for first pset which was to lower case and "whipser":

function whisper(str) {
  return str.toLowerCase();
}

my most recent function which is bouncing between lower and uppercase but the t in test stays lowercase. Getting confused:

function alternatingLetters(str) {
  const arr = str.split("");
  let isUpper = false;

  for (let i = 0; i < arr.length; i++) {
    if (arr[i] !== " ") {
      arr[i] = isUpper ? arr[i].toUpperCase() : arr[i].toLowerCase();
      isUpper = !isUpper;
    }
  }

  return arr.join("");
}

Error message I am getting:

AssertionError: expected 'tHiS iS a TeSt' to equal 'tHiS Is a tEsT'
      + expected - actual

      -tHiS iS a TeSt
      +tHiS Is a tEsT

Any tips would be great! I also tried to make a version without a loop and just return like original solution but not sure if that can be done!

One solution is moving the isUpper flag outside the if condition.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function alternatingLetters(str) {
  const arr = str.split("");
  let isUpper = false;

  for (let i = 0; i < arr.length; i++) {
    if (arr[i] !== " ") {
      arr[i] = isUpper ? arr[i].toUpperCase() : arr[i].toLowerCase();
    }
    isUpper = !isUpper;
  }

  return arr.join("");
}

console.log(alternatingLetters('this is a test'));

<!-- end snippet -->

> I also tried to make a version without a loop and just return like original solution but not sure if that can be done!

Not just with a standard string function (like toLowerCase()), but can be done with map:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function alternatingLetters(str) {
  const arr = str.split("");

  return arr.map(
    (character, i) => (i % 2 === 0)
        ? character.toLowerCase()
        : character.toUpperCase()
  ).join('');
}

console.assert(alternatingLetters('this is a test') === 'tHiS Is a tEsT');

<!-- end snippet -->

function alternatingLetters(str) {
  const arr = str.split("");
  let isUpper = false;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] !== " ") {
      arr[i] = isUpper ? arr[i].toUpperCase() : arr[i].toLowerCase();
    }
    isUpper = !isUpper;
  }
  return arr.join("");
}

try to isUpper = !isUpper; outside of if block.

I clearly misunderstood the question. The function in the following snippet flips the case of each character in the string:

<!-- begin snippet:js console:true -->
<!-- language:lang-js -->

function upsideDown(s){
s=s.split("");
for (let i=0;i<s.length; i++){
let t=s[i].toLowerCase();
s[i]=t==s[i]?s[i].toUpperCase():t;
}
return s.join("");
}

console.log(upsideDown("This Is a cRaZy TeSt!"))
<!-- end snippet -->