Convert a string representation of a comparison operator to an actual comparison operator

Question

I'm trying to create a dynamic function in JavaScript where I can compare one object to another, passing in the comparison operator as a string value to the function.

E.g two objects like this:

{value: 1, name: "banana"}
{value: 2, name: "apples"}

I want to compare banana to apple, is there a way I can pass a string representation of a comparison operator and then use it as an actual comparison operator in a function?

function compare (first, second, comparator) {

    return first.id (comparator) second.id;

}

e.g compare(apple,banana,"<=");
//return true

compare(apple,banana,"===");
//return false

etc

Granted I could implement with a switch or if statement on the comparator string i.e.

 if (comparator === "<=")
    return first.id <= second.id
    if (comparator === "===")
    return first.id === second.id

but I wonder if there is any better more efficient way to do it that avoids the need for such a switch/if statement.

Answer 1

While this might be possible in some languages, JavaScript is not one of them.

Personally I think it's a bad idea, as it comes dangerously close to eval territory. I think you should whitelist the operators and define their behaviour:

switch(comparator) {
    case "<=": return first.id <= second.id;
    case "===": return first.id === second.id;
    // ...
    // you can have synonyms:
    case ">=":
    case "gte": return first.id >= second.id;
    // or even nonexistant operators
    case "<=>": // spaceship!
        if( first.id === second.id) return 0;
        if( first.id < second.id) return -1;
        return 1;
    // and a catch-all:
    default:
        throw new Error("Invalid operator.");
}