How to Connect to localhost Mongodb using an emulator?

Question

First time making a question here so sorry for anything that isn't clear.

Basically I took a react course that set up a functional site with crud and now I'm trying to set up something similar in react native. So I'm new to react, react-native, and mongodb; basically everything I'm working with. This is for self study as well, so just trying to expand my skills.

This is the code that works in my react app under server.js from the backend

const client = await MongoClient.connect("mongodb://localhost:27017", {
      useUnifiedTopology: true,
    });
//api inbetween these two
app.listen(8000, () => console.log("Listening on port 8000"));

On my front end in my package.json I have this

"proxy": "http://localhost:8000/",

from my understanding this basically lets my frontend "proxy"/mask itself as if its coming from port 8000. Then I can call it using a url like below.

useEffect(() => {
    const fetchData = async () => {
      const result = await fetch(`/api/articles/${name}`);
      const body = await result.json();
      setArticleInfo(body);
    };
    fetchData();
  }, [name]);

In my react-native app I get an unhandled promise rejection and in postman I get an internal 500 error with no associated error seems to be null since nothing is printed. Doing some research, I think the issue is because now I'm running this app on an emulator so I think it can't identify the main computer or localhost?

I've tried changing the localhost part of the address to my ipv4 address, to the expo connection url, 0.0.0.0, and also to what I believe is the android localhost 10.0.2.2 but none of them seems to work. example of android localhost change.

server.js

const client = await MongoClient.connect("mongodb://10.0.2.2:27017", {
      useUnifiedTopology: true,
    });
//api inbetween these two
app.listen(8000, () => console.log("Listening on port 8000"));

package.json

"proxy": "http://10.0.2.2:8000/",

When it's set to 10.0.2.2 postman times out and the app still has the unhandled promise error.

When it's set to 0.0.0.0 postman returns a 500 error with no details and the app still has the unhandled promise error.

When it's set to the expo url it straight up doesn't work.

When its set to my ipv4 address I get the unhandled promise and postman returns this error

"error": {
        "name": "MongoServerSelectionError",
        "reason": {
            "type": "Single",
            "setName": null,
            "maxSetVersion": null,
            "maxElectionId": null,
            "servers": {},
            "stale": false,
            "compatible": true,
            "compatibilityError": null,
            "logicalSessionTimeoutMinutes": null,
            "heartbeatFrequencyMS": 10000,
            "localThresholdMS": 15,
            "commonWireVersion": null
        }
    }

Posts I've looked at

Connect to MongoDB Atlas Cluster db with react-native app

Mongodb connection with react native form

React Native / Expo : Fetch throws “Network request failed”

why do we use 10.0.2.2 to connect to local web server instead of using computer ip address in android client

Answer 1

Your useEffect is wriiten wrong

Write it like this

useEffect(() => {
  GetArticles(); // Called whenever there's a change in name
}, [name]);

// Create this function outside useEffect
const GetArticles = async () => {
  try {
    // Also in your fetch you have yo write url like this
    const result = await fetch(
      `http://${YOUR_IPv4_ADDRESS}:8000/api/articles/${name}`
    );
    const body = await result.json();
    setArticleInfo(body);
  } catch (error) {
    console.log(error);
  }
};

Also this is 100% correct

const client = await MongoClient.connect("mongodb://localhost:27017", {
      useUnifiedTopology: true,
    });
//api inbetween these two
app.listen(8000, () => console.log("Listening on port 8000"));

Don't change it to this

const client = await MongoClient.connect("mongodb://10.0.2.2:27017", {
      useUnifiedTopology: true,
    });
//api inbetween these two
app.listen(8000, () => console.log("Listening on port 8000"));