Is It Possible To Set Default Parameter Value On A Rest Parameter

Question

ES6 introduces a bevy of convenient "syntactic sugar". Among them are the default parameter capabilities of JavaScript functions, as well as rest parameters. I'm finding that my console (or devTools) complains (i.e., throws an error) whenever attempting to set a default parameter value on a rest parameter. I've found surprisingly few references to this particular issue elsewhere and am wondering if 1.) it is possible to do so and 2.) why not (assuming it's not possible).

As an example, I've contrived a trivial (but, hopefully still purposeful) example. In this first iteration of the function, I've constructed the function such that it will work (which is to say, without giving the rest parameter a default value).

const describePerson = (name, ...traits) => `Hi, ${name}! You are ${traits.join(', ')}`;

describePerson('John Doe', 'the prototypical placeholder person');
// => "Hi, John Doe! You are the prototypical placeholder person"

However, now with the default:

const describePerson = (name, ...traits = ['a nondescript individual']) => `Hi, ${name}! You are ${traits.join(', ')}`;

describePerson('John Doe');
// => Uncaught SyntaxError: Unexpected token =

Any help is greatly appreciated.

Answer 1

No, rest parameters cannot have a default initialiser. It is not allowed by the grammar because the initialiser would never be run - the parameter always gets assigned an array value (but possibly an empty one).

What you want to do could be achieved by either

function describePerson(name, ...traits) {
     if (traits.length == 0) traits[0] = 'a nondescript individual';
     return `Hi, ${name}! You are ${traits.join(', ')}`;
}

or

function describePerson(name, firstTrait = 'a nondescript individual', ...traits) {
     traits.unshift(firstTrait);
     return `Hi, ${name}! You are ${traits.join(', ')}`;
}

// the same thing with spread syntax:
const describePerson = (name, firstTrait = 'a nondescript individual', ...otherTraits) =>
    `Hi, ${name}! You are ${[firstTrait, ...otherTraits].join(', ')}`

Answer 2

Just came to add a cleaner defaults system:

const describePerson = (name, ...traits) => {
  traits = Object.assign(['x', 'y'], traits);

  return `Hi, ${name}, you are ${traits.join(', ')}`;
}

describePerson('z'); // you are z, y
describePerson('a', 'b', 'c'); // you are a, b, c
describePerson(); // you are x, y

This works because arrays are objects whose indices are keys, and Object.assign overrides the keys of the first object present in the second one with the values of the second one.

If the second one has no index 1, then it won't be overwritten, but if it has index 0, the first array's index 0 will be overwritten by the second, which is the behaviour you expect when defaulting

Note that spreading arrays is not the same operation as spreading objects, which is why [....['x', 'y'], ...traits] wouldn't overwrite the indices