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How to detect if a user is already logged in Firebase with JavaScript?

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Sometimes, we want to detect if a user is already logged in Firebase with JavaScript.

In this article, we’ll look at how to detect if a user is already logged in Firebase with JavaScript.

How to detect if a user is already logged in Firebase with JavaScript?

To detect if a user is already logged in Firebase with JavaScript, we can call the onAuthStateChanged method.

For instance, we write

firebase.auth().onAuthStateChanged((user) => {
  if (user) {
    // ...
  } else {
    // ...
  }
});

to call onAuthStateChanged with a callback that checks if the user is set.

If user is set, then the user is logged in.

Otherwise, the user isn’t logged in.

Conclusion

To detect if a user is already logged in Firebase with JavaScript, we can call the onAuthStateChanged method.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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