Allow either named arguments or positional arguments in Javascript

Question

How can I have a function accept either named arguments (foo({a: 'hello', b: 'it is me'})) or positional arguments (foo('hello', 'it is me'))?

I understand that named arguments can be simulated by passing an object to the function:

function foo(options) {
    options = options || {};
    var a = options.a || 'peanut'; // whatever default value
    var b = options.b || 'butter'; // whatever default value
    console.log(a, b);
}

// ES6 allows automatic destructuring
function foo({a = 'peanut', b = 'butter'} = {}) {
    console.log(a, b);
}

But that does not allow me to accept positional arguments to be passed.

I would like to use ES6 but anything from ES5 would be ok too.

Answer 1

First of all, I really would recommend to stick with one approach. As you said, use either "named"

function foo({a = 'peanut', b = 'butter'} = {}) {
    console.log(a, b);
}

or positional arguments:

function foo(a = 'peanut', b = 'butter') {
    console.log(a, b);
}

Choose the one that fits your function better, do not mix both.

---

If you really need both for some reason, standard overloading techniques are available to you. It'll only work properly if your first positional argument is not an object. I would propose one of the following idioms:

function foo(a, b) { // positional is normal case
    if (arguments.length == 1 && typeof arguments[0] == "object")
        {a, b} = arguments[0];

    console.log(a, b);
}

function foo({a, b}) { // named is normal case
    if (arguments.length > 1 || typeof arguments[0] != "object")
        [a, b] = arguments;

    console.log(a, b);
}

and if you need default values, it gets ugly either way:

function foo(a, b) {
    var opts = (arguments.length == 1 && typeof arguments[0] == "object")
      ? arguments[0]
      : {a, b};
    ({a = 'peanut', b = 'butter'} = opts);

    console.log(a, b);
}

Answer 2

I suppose something like this would work:

function foo(...options){
   if (typeof options[0] === 'object'){
    console.log('expect object', options[0]);
  }else{
    console.log('expect array', options);  
  }
}

foo('peanut', 'butter');
foo({a:'peanut', b:'butter'});