SOURCE

function factorial(n) {
    // debugger;
    if (Number.isInteger(n) && n > 0) {
        if (!(n in factorial)) {
            factorial[n] = n * factorial(n - 1);
        }
        console.log('print:',n);
        return factorial[n];
    } else {
        return NaN;
    }
}

factorial[1] = 1;
console.log('factorial(6):',factorial(6));
console.log('factorial[5]:',factorial[5]);