String manipulations: transform "a-b-c" into "a(b(c))"
Having split the string you can join it with brackets instead of replacing them.  You could optionally choose to remove the repeatString function and your +/- 1, but it does make a lot of sense the way you have it.

function dashesToParentheses(str) {
    var list = str.split('-');
    return list.join('(') + Array(list.length).join(')');
}