Firestore decrement field

const userRef = db.collection('documents').doc('document_id');
const increment = firebase.firestore.FieldValue.increment(1);

userRef.update({ FieldToIncrease: increment });

const userRef = db.collection('documents').doc('document_id');
const decrement = firebase.firestore.FieldValue.increment(-1);

userRef.update({ fieldToDecrease: decrement });