NodeJS mongoClient invalid operator: $eq

I have a mongo property (category), where I want to filter with equals. I used the following syntax:

{"category":{"$eq":"cat1"}}

(I know, it can be written like:

{"category":"cat1"}

If I use the $eq operator, the mongoClient fails with "invalid operator: $eq". But only on localhost. If I deploy the same code to a remote server, it works perfectly.

How can I make it work on localhost? It would be preferred to use the $eq operator - there are many places in the code where this is used.

Thx