Capitalizes the first letter of a string.

Use array destructuring and String.prototype.toUpperCase() to capitalize first letter, to get array of characters after first letter and then Array.prototype.join('') to make it a string again.
Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lowercase.


const capitalize = ([first,], lowerRest = false) =>
  first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));


capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'