function countLetterOccurence(str, letter) {
    const reg = str.match(new RegExp(letter, 'g'));
    return (reg ? reg : []).length;
}

const text = 'Hey my name is Eric, Im a front-end developer and mostly do my work on a laptop';
console.log(`Letter a is mentioned $(countLetterOccurence(text, 'a')} times`);