How to increment existing number field in Cloud Firestore

Firestore now has a specific operator for this called FieldValue.increment(). By applying this operator to a field, the value of that field can be incremented (or decremented) as a single operation on the server.

From the Firebase documentation on incrementing a numeric value:

var washingtonRef = db.collection('cities').doc('DC');

// Atomically increment the population of the city by 50.

washingtonRef.update({
    population: firebase.firestore.FieldValue.increment(50)
});

So this example increments the population by 50, no matter what is current value is. If the field doesn't exist yet (or is not a number), it is set to 50 in this case.

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If you want to decrement a number, pass a negative value to the increment function. For example: firebase.firestore.FieldValue.increment(-50)