is-vowel

Check if character is a vowel

const isVowel = char => /[aeiou]/i.test(char);

It could be used in a recursion to count vowels in a string

const countVowels = str => !str?.length ? 0 : isVowel(str[0]) + countVowels(str.slice(1));

Regex breakdown

• / regex boundary
• [] group bracket expression matching single chars it contains
• aeiou chars to test against
• i case insensitive flag