Use variable in Javascript file

What is the correct way of using variables from Javascript files?

Suppose I have one variable that is a resource ID, then I use this variable to find all other resources related to the first resource.

I was able to achieve it by looking at the implementation, but I believe it is not a safe way. For example:

serverless.yml:

service: example

provider:
  name: aws
  runtime: nodejs12.x

custom:
  example: ${opt:example, "fallback"}
  reversed: ${file(loadVars.js):reverse}

functions:
  helloWorld:
    handler: handler.helloWorld
    environment:
      REVERSED: ${self:custom.reversed}
    events:
      - http:
          path: hello-world
          method: get
          cors: true

loadVars.js:

async function reverse (serverless) {
    const exampleVar = serverless.service.custom.example
    const exampleValue = await serverless.variables.populateValue(exampleVar, true)

    if (!exampleValue) {
        throw new Error("Not defined")
    }

    return exampleValue.split('').reverse().join('')
}

module.exports = {
    reverse
}

no one? I will leave as it is.

I don’t know about this issue, hope someone will help you answer.