Make the first letter of a string uppercase in JavaScript

capitalizeFirstLetter.js

const capitalizeFirstLetter = s => {
  const type = typeof s;
  if (type !== 'string') {
    throw new Error(`Expected string, instead received ${type}`);
  }

  const [firstChar, ...remainingChars] = s;

  return [firstChar.toUpperCase(), ...remainingChars].join('');
};