---
id: 5895f70cf9fc0f352b528e66
title: Serialization of a User Object
challengeType: 2
forumTopicId: 301563
dashedName: serialization-of-a-user-object
---

# --description--

Serialization and deserialization are important concepts in regards to authentication. To serialize an object means to convert its contents into a small *key* that can then be deserialized into the original object. This is what allows us to know who has communicated with the server without having to send the authentication data, like the username and password, at each request for a new page.

To set this up properly, we need to have a serialize function and a deserialize function. In Passport, we create these with `passport.serializeUser( OURFUNCTION )` and `passport.deserializeUser( OURFUNCTION )`

The `serializeUser` is called with 2 arguments, the full user object and a callback used by passport. A unique key to identify that user should be returned in the callback, the easiest one to use being the user's `_id` in the object. It should be unique as it is generated by MongoDB. Similarly, `deserializeUser` is called with that key and a callback function for passport as well, but, this time, we have to take that key and return the full user object to the callback. To make a query search for a Mongo `_id`, you will have to create `const ObjectID = require('mongodb').ObjectID;`, and then to use it you call `new ObjectID(THE_ID)`. Be sure to add `mongodb@~3.6.0` as a dependency. You can see this in the examples below:

```js
passport.serializeUser((user, done) => {
  done(null, user._id);
});

passport.deserializeUser((id, done) => {
  myDataBase.findOne({ _id: new ObjectID(id) }, (err, doc) => {
    done(null, null);
  });
});
```

NOTE: This `deserializeUser` will throw an error until we set up the DB in the next step, so for now comment out the whole block and just call `done(null, null)` in the function `deserializeUser`.

Submit your page when you think you've got it right. If you're running into errors, you can check out the project completed up to this point [here](https://gist.github.com/camperbot/7068a0d09e61ec7424572b366751f048).

# --hints--

You should serialize user function correctly.

```js
(getUserInput) =>
  $.get(getUserInput('url') + '/_api/server.js').then(
    (data) => {
      assert.match(
        data,
        /passport.serializeUser/gi,
        'You should have created your passport.serializeUser function'
      );
      assert.match(
        data,
        /null,\s*user._id/gi,
        'There should be a callback in your serializeUser with (null, user._id)'
      );
    },
    (xhr) => {
      throw new Error(xhr.statusText);
    }
  );
```

You should deserialize user function correctly.

```js
(getUserInput) =>
  $.get(getUserInput('url') + '/_api/server.js').then(
    (data) => {
      assert.match(
        data,
        /passport.deserializeUser/gi,
        'You should have created your passport.deserializeUser function'
      );
      assert.match(
        data,
        /null,\s*null/gi,
        'There should be a callback in your deserializeUser with (null, null) for now'
      );
    },
    (xhr) => {
      throw new Error(xhr.statusText);
    }
  );
```

MongoDB should be a dependency.

```js
(getUserInput) =>
  $.get(getUserInput('url') + '/_api/package.json').then(
    (data) => {
      var packJson = JSON.parse(data);
      assert.property(
        packJson.dependencies,
        'mongodb',
        'Your project should list "mongodb" as a dependency'
      );
    },
    (xhr) => {
      throw new Error(xhr.statusText);
    }
  );
```

Mongodb should be properly required including the ObjectId.

```js
(getUserInput) =>
  $.get(getUserInput('url') + '/_api/server.js').then(
    (data) => {
      assert.match(
        data,
        /require.*("|')mongodb\1/gi,
        'You should have required mongodb'
      );
      assert.match(
        data,
        /new ObjectID.*id/gi,
        'Even though the block is commented out, you should use new ObjectID(id) for when we add the database'
      );
    },
    (xhr) => {
      throw new Error(xhr.statusText);
    }
  );
```

# --solutions--

```js
/**
  Backend challenges don't need solutions, 
  because they would need to be tested against a full working project. 
  Please check our contributing guidelines to learn more.
*/
```
