JavaScript: CamelCase string conversion to Proper Case

I ran into a case where I wanted to be able to reverse a CamelCase string to Proper Case format. Without using regex, due to the amount of time it would take for regex to make the replacement, I built up a function to convert a CamelCased string with the most likely proper result possible.

This function will add a space before a capital letter if it is not the first letter in the sentence, if the following character is not a capital (allow abbreviations/acronyms) and if the previous character was not a space (unlikely to be used, but allows for partial camel cased strings to be corrected). Unfortunately this function will not be able to detect a single letter capital word as an independent work, like 'I'. Since it's less common that 'I' would be used in a variable, when compared to an acronym or abbreviation, this was decided to be the lesser evil of errors.

String.prototype.camelToProper = function () {
 if (this == null || this == "") {
  return this;
 }
 var newText = "";
 var characters = this.split("");
 for (var i = 0; i < characters.length; i++) {
  if (characters[i] == characters[i].toUpperCase()
 && i != 0
 && !(characters[i + 1] == characters[i + 1].toUpperCase())
 && characters[i - 1] != " ") {
 newText += " ";
  }
  newText += characters[i];
 }

 return newText;
}```