How to get all the suggested string on the basis of search string in javascript

I have two string, one which contains all the values of countries and second string which I entered when I’m searching for country. Now, I want all the results which contains "In" like result will be
India, Indonesia

as I have search for IN

// s1="In", s2="countries value"

const s2 = "India, Australia,Canada,Luxembourg,Singapore,Switzerland,United States of America, Indonesia";
const s1 = "In"

const findString = (s1, s2) => {
if(s2.includes(s1.charAt(0).toUpperCase() + s1.slice(1))){
  return s1;
} else {
  return null;
}

}

this function returning me "IN" but I want "India, Indonesia" as result

>Solution :

const s2 = "India, Australia,Canada,Luxembourg,Singapore,Switzerland,United States of America, Indonesia";
const s2Arr = s2.replace(/, +/g, ',').split(',') // create array all country

function findString(keyword) {
  keyword = keyword.toLowerCase()

  return s2Arr.filter(item => item.toLowerCase().includes(keyword))
}