How to increment a record in Firebase?

You can use transaction

var databaseRef = firebase.database().ref('users').child(user_uid).child('searches');

databaseRef.transaction(function(searches) {
  if (searches) {
    searches = searches + 1;
  }
  return searches;
});

For the Realtime Database the use of transaction seems to be the best way to do it. See the answer from Sunday G Akinsete

From his answer and the related comments:

firebase
    .database()
    .ref('users')
    .child(user_uid)
    .child('searches')
    .transaction(function(searches) {
        return (searches || 0) + 1
    })

For the Firestore Database you can use the Firebase Sentinel values to increment/decrement a value that without having to fetch it first

Increment

firebase
    .firestore()
    .collection('users')
    .doc('some-user')
    .update({ 
         valueToIncrement: firebase.firestore.FieldValue.increment(1) 
    })

Decrement

firebase
    .firestore()
    .collection('users')
    .doc('some-user')
    .update({ 
        valueToDecrement: firebase.firestore.FieldValue.increment(-1) 
    })

Documentation Reference

There's a new method ServerValue.increment()in firebase JavaScript SDK v7.14.0

It's better for performance and cheaper since no round trip is required.

See here

Added ServerValue.increment() to support atomic field value increments without transactions.

API Docs here

Usage example:

firebase.database()
    .ref('users')
    .child(user_uid)
    .child('searches')
    .set(firebase.database.ServerValue.increment(1))

Or you can decrement, just put -1 as function arg like so:

firebase.database()
    .ref('users')
    .child(user_uid)
    .child('searches')
    .set(firebase.database.ServerValue.increment(-1))