Recursive re-implementation of JSON.stringify

stringify.js

function stringify(obj, seenStack = []) {
  if (obj instanceof Array) {
    return "[" +
           obj.reduce((acc, v, idx) => {
             let sanitizedV = v;
             if (typeof sanitizedV === 'string') {
               sanitizedV = "\"" + sanitizedV + "\"";
             }
             return acc + (idx !== 0 ? "," : "") + stringify(sanitizedV, seenStack);
           }, "") +
           "]";
  } else if (typeof obj === 'object' && obj !== null) {
      return "{" + 
              Object.entries(obj).reduce((acc, [k, v], idx) => {
                if (typeof v !== 'undefined') {
                  if (seenStack.indexOf(v) !== -1) {
                    return acc + '[circular reference]';
                  }
                  return acc + 
                         (idx !== 0 ? "," : "") + 
                         "\"" + k + "\":" + 
                         stringify(v, (seenStack.push(v) && seenStack));
                } else {
                  return acc;
                }
              }, "") + 
              "}";
  } else if (typeof obj !== 'undefined') {
    return "" + obj;
  } else {
    return "";
  }
}

const test1 = {"a": 5, "b": null, "c": undefined};
console.log(stringify(test1), 'should be', JSON.stringify(test1));

const test2 = {"a": 5, 
               "b": {"d": 23,
                     "e": 24}, 
               "c": 99};
console.log(stringify(test2), 'should be', JSON.stringify(test2));

const test3 = 8;
console.log(stringify(test3), 'should be', JSON.stringify(test3));


const test4 = {};
const test5 = {"b": test4,
               "c": 8,
               "d": {"e": 9,
                     "f": {"g": 10}}};
test4.a = test5;
console.log(stringify(test4));
/* JSON.stringify will throw error here (at least if running in jsbin.com)
console.log('should be', JSON.stringify(test4));
*/

const test6 = ['a', 'b', 8, test2];
console.log(stringify(test6), 'should be', JSON.stringify(test6));

// possible improvements:
// dates differently?