An implementation of JSON.stringify

stringify.js

function stringifyObj(obj) {
  let toStr = Object.prototype.toString;
  if(toStr.call(obj) == "[object String]") {
    return "\"" + String(obj) + "\"";
  } else if (toStr.call(obj) == "[object Number]") {
    return String(obj);
  } else if(toStr.call(obj) == "[object Array]") {
    let resultStr = "["
    for (let i = 0; i < obj.length; i++) {
      resultStr += stringifyObj(obj[i]);
      if (i === obj.length - 1) continue;
      resultStr += ","
    }
    return retStr + "]";
  } else if(toStr.call(obj) == "[object Object]") {
    obj = hashify(obj);
    let keys = Object.keys(obj),
        resultStr = "{";
    for (let i = 0; i < keys.length; i++) {
      resultStr += "\"" + keys[i] + "\":" + stringifyObj(obj[keys[i]]);
      if (i == keys.length - 1) continue;
      resultStr += ",";
    }
    return resultStr + "}"
  }

  function hashify(obj) {
    let resultObj = {},
        keys = Object.keys(obj);
    
    for (let i=0; i < keys.length; i++) {
      if (typeof obj[keys[i]] !== "function") {
        resultObj[keys[i]] = obj[keys[i]];
      }
    }
    return resultObj;
  }
}