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String: indexOf() with from index

The indexOf method allows you to specify the position from which the search should start. This is done with a second parameter.
var n1 = 'White Rabbit'.indexOf('it', 1);
var n2 = 'White Rabbit'.indexOf('it', 3);
The first 'it' in 'White Rabbit' has the index 2. Starting our search at position 1 it will be found. n1 is 2. The second 'it' in 'White Rabbit' has the index 10. Starting our search at position 3 it will be found. n2 is 10.


Write a function secondIndexOf, taking two strings and determining the second occurrence of the second string in the first string. If the search string does not occur twice, -1 should be returned. secondIndexOf('White Rabbit', 'it') should return 10.
var secondIndexOf = function(s1, s2) {
// Use indexOf twice. 
var secondIndexOf = function(s1, s2) {
  var firstIndex = s1.indexOf(s2);
  return s1.indexOf(s2, firstIndex + 1);