longest subsequence between two strings

longest-subsequence.js

/*
Given two strings s1 and s2, return the longest common subsequence of s1 and s2 
(with longest common subsequence defined as the longest sequence of characters 
such that all of them appear in both of the strings, possibly with other characters in between)
'ABAZDC' 'BACBAD' => ABAD
'AGGTAB' 'GXTXAYB' => GTAB
'aaaa' 'aa' => aa
*/

console.log(getLongestSub('ABAZDC','BACBAD'));
console.log(getLongestSub('AGGTAB','GXTXAYB'));
console.log(getLongestSub('aaaa','aa'));

function getLongestSub(str1, str2, memo = []) {
    if (!str1.length || !str2.length) return '';
    let first, second;

    if ( str1.length >= str2.length ) 
    { 
        first = str1; 
        second = str2; 
    } else { 
        first = str2; 
        second = str1; 
    }
    const letter = first.charAt(0);
    const idx = second.indexOf(letter);

    if( idx >= 0 ) {
        memo.push(letter);
        getLongestSub(first.substring(1), second.substring(idx + 1), memo);
    } else {
        getLongestSub(first.substring(1), second, memo);
    }

    return memo.join('');
}