// use strict mode, more errors will show this way. 
"use strict";

var capitalize = function(element) {
  // get the inner Text and turn it into an array, spliting all the spaces
  var text = element.innerText.split(' '),
      // result string
      result = '';
  
  // for every element of the new Text array
  text.forEach(function(word) {
    // get the first letter
    var firstLetter = word.slice(0,1),
        // and all remaining letters
        remaining = word.slice(1);
    // then simply use toUpperCase() on 
    // the first letter and add back the remaining part, also
    // add a space to the end of the string
    result += firstLetter.toUpperCase() + remaining + ' ';
  });
  // set the elements inner text to the result
  element.innerText = result;
  // return the result, this way it can also be easily logged using console.log()
  return result;
}

var demoText = document.querySelector('#target'),
    demoTextSingle = document.querySelector('#single');

capitalize(demoText);
capitalize(demoTextSingle);