count.js

function countChar(stringOfCharacters, findCharacter) {
  if (typeof stringOfCharacters != "string") {
    throw new Error("Input should be a string");
  }
  if (typeof findCharacter !== "string" || findCharacter.length != 1) {
    throw new Error("Input must be a single character");
  }

  let count = 0;

  for (let char of stringOfCharacters) {
    if (char === findCharacter) {
      count++;
    }
  }
  return count;
}

module.exports = countChar;

console.log(countChar("amazon", "a"));

// Added lines to check cases and return count of characters.
// Indentation improved.
// count.js formatted with prettier.