Idiomatic way to make middle arguments optional

Question

What is a common way to implement a JavaScript function whose middle argument(s) are optional, but the last argument is required? An idiomatic ES2015 solution is preferred.

E.g., I commonly see functions that can be invoked in multiple ways, like this:

func(firstArg, callback);
func(firstArg, options, callback);

So what actually happens is that the number of arguments affects the interpretation of each argument.

I know I can handle this by checking the length of the arguments, something like this:

//     func(firstArg, options, callback);
// or: func(firstArg, callback);
function (firstArg, ...args) {
    let options = {};
    if (args.length > 1) {
        options = args.shift();
    }
    let callback = args[0];
    // And do stuff ...
}

But it seems clunky. Is there a better way to do this?

Answer 1

Use ...args and pop() the callback from the array:

function thing(...args) {
  var cb = args.pop();
  //... args is now [1, obj]
  cb(args[0]);
}

thing(1, { a: 1 }, function (data) {
  console.log(data); // 1
});