How to store and retrieve JavaScript function stored in MongoDB document?

Question

I have an "Assignments" MongoDB collection with documents containing a scoring function. I want to store this scoring function in the Assignment document as the scoring logic may change over time as new assignments are created:

Assignment

{
  _id: "assignment1",
  question: "Number between 1 and 10",
  scoringFunction: function (value) { value * 2 }
}

When a user completes an assignment a new document in the "Events" collection is inserted:

Event

{
  _id: "event1",
  answer: "4",
  score: "8"
}

I can insert the above Assignment document into my Assignments collection, however when I attempt to retrieve the scoringFunction it's no longer a function it's an object.

Some debugging code:

const assignment = Assignments.findOne({ _id: "test" })
console.log(assignment.scoringFunction)

Debugging code output:

Code {
  _bsontype: 'Code',
  code: 'function (value) { value * 2 }',
  scope: undefined
}

How can I decode this object back into a JavaScript function which can be called with parameters to calculate the score?

Answer 1

Warning: Avoid storing executed code along with your data.

Ideally — from a data security perspective — don’t store executable code in your data.

In the case that a hacker (or user) is able to alter this function in your database, they could compromise your data or break your app if the function doesn’t work correctly.

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Implemented Solution

I'm going to store the name of the scoring function in the assignment.

Assignment

{
  _id: "assignment1",
  question: "Number between 1 and 10",
  scoringFunction: "scoreAssignment1"
}

Meteor Logic

const scoreAssignment1 = (value) => value * 2
const assignment = Assignments.findOne({ _id: "test"})
console.log(Meteor.call(assignment.scoringFunction, 2))    
// output: 4

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Dangerous Solution

Store the body of the scoringFunction in a string, then use new Function() to create a function from the desired parameters and function body string.

Assignment

{
  _id: "assignment1",
  question: "Number between 1 and 10",
  scoringFunction: "value * 2"
}

Event Logic

const assignment = Assignments.findOne({ _id: "test"})
const scoringFunction = new Function('value', assignment.scoringFunction)
console.log(scoringFunction(2))
// output: 4