is $lookup required (in lieu of square brackets ala js) to dynamically select object properties?

[darrencruse]

i.e. object properties whose names aren't known until run time?

to clarify: in plain javascript if I have something like this:

const json = {
  "esFields": {
    "param1": "esField1.keyword",
    "param2": "esField2.keyword"
  }
}

it's typical to do just json.esFields.param1 if u know the "param1" part ahead of time
but if if u have a variable param whose value is the "param1" then (idiomatically) u switch to json.esFields[param]

In trying to use JSONata to generate some elasticsearch query json I struggled where I was trying to map input parameter names to elasticsearch field names using an object to do the mapping

This is a simplified equivalent of what I was attempting and the error I was getting (where I realized the square brackets seemingly do not/cannot just take a property name string?)
can this be made to work without $lookup?

I did finally get it working using $lookup like this:
working with $lookup

(but in my mind what I was doing seemed simple mostly I just wanted to confirm there wasn't a simple solution that didn't use the $lookup)

[markmelville]

I recently asked @andrew-coleman about this, and he said "I'm running out of tokens." My question, Andrew, is: can a token be interpreted differently depending on context:
$[$prop]
Could the [] token filter the array if the input is an array, and map to the property if it's an object? Currently if the argument is a string it always evaluates true, and so passes the object through no matter the value.

[darrencruse]

Yeah if it could (like u say) apply a rule that if the input before the square brackets is an object and what's inside the predicate square brackets evaluates to a string then effectively do the $lookup(input, predicateString) it feels like that would help the usability a lot (for people used to javascript!!)