Euler #25 Scala

I usually write Euler solutions in Javascript. On problems requiring very large integers I have decided to learn Scala and take advantage of BigInt. I wrote a solution to Euler025, it works fast but reads like Javascript. Will the reviewer please comment on how to write this solution more idiomatically and, if you have a JS background, what resources did you find useful to learn syntax/best practices?

What is the index of the first term in the Fibonacci sequence to contain 1000 digits?

Solution:

def e25(): Int = {
  var a = BigInt("1")
  var b = BigInt("1")
  var index = 3

  while(index > 0) {
    val temp = a + b
    if (temp.toString().length() > 999) {
      return index
    }
    index += 1
    a = b
    b = temp
  }
  index
}

println(e25())