Javascript Line break detection and line break preservation

Question

I wrote a function to capitalize each letter at the beginning of each sentence. I want to output the text that originally has line breaks to a text-area from a text-area. The output text should preserve these line breaks. But my code is ignoring all the linebreaks. Can you help me with this? My code:

btnPara.addEventListener("click", (e) => {
  e.preventDefault();

  function paragraphMode(str) {
    var splitPara = str
      .split(/\r?\n/)
      .join(" ")
      .split(". ")

      .map(
        (sentence) => sentence.charAt(0).toUpperCase() + sentence.substring(1)
      );

    return splitPara.join(". ");
  }
  outputText.value = paragraphMode(inputText.value);
});

Answer 1

You don't need to process newline, you only need to find all occurrences of first char after a .. For example, you can use /(^|\.)\s*(\w)/g

const process = s => s.replace(/(^|\.)\s*(\w)/g,w=>w.toUpperCase())

console.log(process("abc.    def .  g   h . ijk lm. \n O \n pq\nR\ns"))
console.log(process("some text is\nwritten here.  it doens't\nproperly capitalize sentences.\ncan we fix it?")) // example sentence suggested by @ Scott Sauyet

---

btw, for the same effect of your origin code (only one space/newline after a period is matched), you simply change \s* to \r?[\n ]

const input = "abc.    def .  g   h . ijk lm. \n O \n pq\nR"
const result = input.replace(/(^|\.)(\r?\n| )(\w)/g,w=>w.toUpperCase())
console.log(result)