javascript string reverse
"this is a test string".split("").reverse().join("");
//"gnirts tset a si siht"
// Or
const reverse = str => [...str].reverse().join('');
// Or
const reverse = str => str.split('').reduce((rev, char)=> `${char}${rev}`, '');
// Or
const reverse = str => (str === '') ? '' : `${reverse(str.substr(1))}${str.charAt(0)}`;
// Example
reverse('hello world'); // 'dlrow olleh'