recursively capitalize a string that is separated by underscores:

capitalize.js

function capitalize(s){
  s = s || [''];
  var titleCase = s[0].toUpperCase() + s.slice(1);

  var idx = titleCase.indexOf("_");
  
  if (idx != -1){
    first = titleCase.slice(0, idx);
    last = titleCase.slice(idx+1);
    return first + ' ' + capitalize(last);
  }

  return titleCase;
  
}

console.log(capitalize("mark_james_feltner")); // Mark James Feltner