Firebase HTTP Cloud Function error 'Cannot GET /' - node.js

Question

I am trying to get users data from firebase but I keep receive 'CANNOT GET /' error.
//firebase init
const functions = require('firebase-functions');
const admin = require("firebase-admin");
admin.initializeApp();
//express and cors init
const express = require('express');
const cors = require('cors');
//middleware init
const app = express();
app.use(cors());
const database = admin.database();
app.get("/users", function (request, response) {
return database.ref('/users').on("value", snapshot => {
return response.status(200).send(snapshot.val());
}, error => {
console.error(database);
return response.status(500).send(err);
})
});
exports.users = functions.https.onRequest(app);
I also try to use another function from another website I refer to get at least the username but it returns error.
exports.users = functions.https.onRequest((req, res) => {
database().ref('/merchants').once('value').then(function(snapshot) {
var username = snapshot.val().username;
res.status(200).send(username);
});
});

Answer 1

Problem 1: Exporting express applications
When you export an express app through a Cloud Function, the paths become relative to the exported function name. You exported your app as exports.users which sets the root path of your function to /users and to call it, you would visit https://us-central1-<project-id>.cloudfunctions.net/users.
However, because you defined a route handler for /users (using app.get("/users", ...)) as well, you added a handler for https://us-central1-<project-id>.cloudfunctions.net/users/users instead.
The error Cannot GET / is thrown because when you call the function at https://us-central1-<project-id>.cloudfunctions.net/users, the relative URL is set as "/", which you haven't configured a handler for (using app.get("/", ...)).
So to fix your code above, change
app.get("/users", function (request, response) {
to
app.get("/", function (request, response) {
Problem 2: Username queries
The issue could be as simple as that you call database() instead of admin.database().
However, the purpose of this query is unclear, so I will assume that you have a merchant's ID and you are trying to get the username of the owner of that particular merchant.
This would mean a data structure similar to:
{
"users": {
"somePerson": { ... },
"otherPerson: { ... }
},
"merchants": {
"reallyGreatGuyInc": {
"username": "somePerson",
...
},
"realShadyPeopleInc": {
"username": "otherPerson",
...
}
}
}
If you exported a function called getMerchantUsername where you pass in the merchant ID via a GET parameter and call it at the URL https://us-central1-<project-id>.cloudfunctions.net/getMerchantUsername?id=reallyGreatGuyInc, you would define it using the following code:
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
exports.getMerchantUsername = functions.https.onRequest((req, res) => {
if (!req.query.id) {
// '?id=' is required
return res.status(400).send("Missing 'id' parameter");
} else if (/[\u0000-\u001F\u007F\.\$#/\[\]\\]/.test(req.query.id)) {
// check id for invalid characters
return res.status(400).send("Invalid 'id' parameter");
}
// run query
admin.database().ref(`/merchants/${req.query.id}/username`).once('value')
.then((snapshot) => {
var username = snapshot.val();
res.status(200).send(username);
})
.catch((error) => {
console.error(error);
res.status(500).send('Request failed.');
});
});
Notes:
The test for invalid characters is based on the list in the RTDB: Structure data documentation. This somewhat prevents looking at other database values.
If you only need the value of username, rather than request the entire merchant object and parse it, request only the username as in the example above where I used `/merchants/${req.query.id}/username`.