.filter is not a function 

filter is a method on arrays. Since the code you posted contains an object, you’re seeing this error. You may want to apply filter after getting all the values from the object using Object.values, like this:

var users = {
  "1": {
    "user_id": 1,
    "test": "",
    "user_name": "potato0",
    "isok": "true"
  },

  "2": {
    "user_id": 2,
    "test": "",
    "user_name": "potato1",
    "isok": " true"
  },

  "3": {
    "user_id": 3,
    "test": "",
    "user_name": "potato2",
    "isok": " true"
  },

  "4": {
    "user_id": 4,
    "test": "",
    "user_name": "potato3",
    "isok": "locationd"
  }
};

console.log(Object.values(users).filter(user => user.user_id === 1));