type DocumentData = { [field: string]: any };
let data1: DocumentData = {4:3};
console.log(data1); //{4:3}
It appears that the DocumentData type in the code above defines an object type where the key is of string type and the value can be of any type. This is similar to the Map<String, dynamic> type in Dart, which I am familiar with.
Surprisingly, when I tested the code on the following website, no errors were encountered. Could it be that I am misunderstanding something?
Despite the explanation above, I still find it difficult to comprehend the exact data structure represented by the DocumentData type.