simple binary search

binarysearch.js

var bsearch = function(f, a, b, z) {
  if (a + 1 === b) {
    return a;
  }
  var m = Math.floor(( a + b ) / 2);
  if (f(m) <= z) {
    return bsearch(f, m, b, z);
  } else {
    return bsearch(f, a, m, z);
  }
}

bsearch(R.nth(R.__, [0,4,6,8,9,10,19]), 0, 6, 1) // 0