repeat.go

func Repeat(s string, n uint) string {
	switch n {
	case 0:
		return ""
	case 1:
		return s
	}

	max := len(s) * int(n)
	var b strings.Builder
	b.Grow(max)
	b.WriteString(s)
	for ; 2 <= n; n = n >> 1 {
		b.WriteString(b.String())
	}
	b.WriteString(b.String()[:max-b.Len()])
	return b.String()
}

repeat.js

const repeat = (s, n) => {
  if (n === 0) return '';
  else if (n === 1) return s;
  const max = s.length * n;
  for (; 2 <= n; n = n >> 1) { s += s; }
  return s + s.substring(0, max - s.length);
}