Create a unique user id in Firebase

Let's assume that we want to create a unique identification number for a user (uid) in Firebase.

Numerical

Like in Stack Overflow, the straightforward way is to create a counter in the database and assign the last value plus one to a new user.

Example:

javascript
const getUid = async () => {
    
    let newUid;
    
    await admin.database().ref('/users/counter').transaction(value => {
        
        newUid = value + 1;
        
        return (value + 1);
        
    });
    
    return newUid;
    
}

Math.random()

Alternatively, we could create a unique identification number by concatenating 4 random strings of 4 characters.

Example:

javascript
const getUid = () => {
    
    let wwww = Math.random().toString(32).slice(-4);
    let xxxx = Math.random().toString(32).slice(-4);
    let yyyy = Math.random().toString(32).slice(-4);
    let zzzz = Math.random().toString(32).slice(-4);
    
    return wwww + xxxx + yyyy + zzzz;
    
}

If we try to launch this function 1000000 times, let's see how many duplicates we get:

javascript
let random = {};
let duplicates = 0;

for(let i = 0; i < 1000000; i ++){
    
    let result = getUid();
    
    if(!random[result])
        random[result] = true;
    else
        duplicates ++;
    
}

console.log(duplicates);
// 0

There are no duplicates for 1000000 users. Feel free to try the function with more iterations to check if there are collisions. If you encounter collisions, you could add more random chars to the getUid() function.