MongoDB supports Schema Validation since MongoDB 3.2 (Dec. 2015) and JSON Schema since MongoDB 3.6 (Nov. 2017) so MongoDB can enforce types strictly if you want to at the collection level.
NoSQL doesn't mean NoSchema or NoTypes.
https://docs.mongodb.com/manual/core/schema-validation/#json...
So they bolted it on eventually then. The relational databases went the other way and bolted-on schemaless data [0] [1]. We can still compare the schema-based and schemaless approaches.
[0] https://www.postgresql.org/docs/10/datatype-json.html
[1] https://docs.microsoft.com/en-us/sql/t-sql/functions/json-qu...