camelCase to kebab-case

I have a kebabize function which converts camelCase to kebab-case. I am sharing my code. Can it be more optimized? I know this problem can be solved using regex. But, I want to do it without using regex.

const kebabize = str => {

    let subs = []
    let char = ''
    let j = 0

    for( let i = 0; i < str.length; i++ ) {

        char = str[i]

        if(str[i] === char.toUpperCase()) {
            subs.push(str.slice(j, i))
            j = i
        }

        if(i == str.length - 1) {
            subs.push(str.slice(j, str.length))
        }
    }

    return subs.map(el => (el.charAt(0).toLowerCase() + el.substr(1, el.length))).join('-')
}

kebabize('myNameIsStack')

const kebabize = str => {
   return str.split('').map((letter, idx) => {
     return letter.toUpperCase() === letter
      ? `${idx !== 0 ? '-' : ''}${letter.toLowerCase()}`
      : letter;
   }).join('');
}

console.log(kebabize('myNameIsStack'));
console.log(kebabize('MyNameIsStack'));

You can just check every letter is if upperCase or not and replace it.