Exploring the concept of 'JavaScript with Scope' within the MongoDB environment

Within the link provided at https://docs.mongodb.com/manual/reference/bson-types/, it discusses JavaScript with Scope as a potential data type found in documents.

I have some inquiries:

(1) Could you explain what exactly JavaScript with scope entails?

(2) Is this considered an "Internal" data type within MongoDB?

When I say "internal", I am referring to a type that cannot be utilized by users. Unfortunately, I haven't been able to locate more detailed information regarding this particular type aside from its mention in the aforementioned link.

(3) While using the mongo C driver, I came across Struct bson_value_t at . Can you provide insight into what the "scope_data" buffer represents?

javascriptmongodbbsonmongo-c-driver

Answer №1

Did you know that you can actually save a JavaScript function in a MongoDB collection and execute it from there?

> db.collection.insert({ name: "add1", f: (function(x) { return x + 1 }) })
WriteResult({ "nInserted" : 1 })
> db.collection.findOne({ name: "add1" }).f(123)
124

What is commonly known as a "closure" is a function that makes reference to variables outside of its own scope, as shown with the function incrementX:

var x = 1;
function incrementX() { x++; }

Surprisingly, these functions can also be stored in a MongoDB collection and will interact with the current session's environment:

> db.collection.insert({
    name: "incrementX",
    f: (function() { x++; })
})
WriteResult({ "nInserted" : 1 })
> var x = 123;
> db.collection.findOne({ name: "incrementX" }).f()
> x
124

Interestingly, the BSON data type used for storing JavaScript functions varies based on whether they involve closure or not. The plain "JavaScript" type is utilized for functions without closures, while "JavaScript (with scope)" is used for those with closures.

The rationale behind saving a JavaScript function in a MongoDB collection may be unclear. It does pose some risks and limitations, especially if accessed through non-JavaScript drivers or when facing potential security vulnerabilities from user-injected functions. Perhaps it's best to overlook this feature and proceed cautiously.

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