JS function to convert strings to title case and ignore certain reserved words such as "a", "an", etc.

to-title-case.js

function toTitleCase(string) {
  const convertTitleCase = (str) => `${str.charAt(0)}${str.toLowerCase().slice(1)}`
  const reservedWords = new Set(["a", "an", "at", "in"]);
  return string
    .split(" ")
    .map((s) => {
      if (reservedWords.has(s)) {
        return s.toLowerCase();
      }
      return convertTitleCase(s);
    }).join(" ");
}