FieldPath
interface
A FieldPath refers to a field in a document. The path may consist of a single field name (referring to a top-level field in the document), or a list of field names (referring to a nested field in the document).
Create a FieldPath by providing field names. If more than one field name is provided, the path will point to a nested field in a document.
Example
const user = await firebase.firestore().doc('users/alovelace').get();
// Create a new field path
const fieldPath = new firebase.firestore.FieldPath('address', 'zip');
console.log('Address ZIP Code', user.get(fieldPath));