JavaScript Rest Parameters - Exercise 1

A rest parameter collects every argument that follows the named parameters into a new array. This gives the function a clear name for processing a flexible number of values.

function logAll(...values) {
  console.log(values);
}

A rest parameter must be the final parameter because it collects all remaining arguments. A later parameter could not be assigned reliably, so function f(...rest, last) {} is invalid.

function join(separator, ...parts) {
  return parts.join(separator);
}

Yes. The collected values are a real array, so properties such as length and methods such as map are available without conversion.

function count(...items) {
  return items.length;
}

Rest parameters replace most uses of the older arguments object by providing named array access to the remaining arguments. Unlike arguments, the rest value is a true array.

function sum(...numbers) {
  return numbers.reduce((total, value) => total + value, 0);
}

No. A parameter list can contain only one rest parameter, and it must be last.

function collect(first, ...rest) {
  return [first, rest];
}

In an array pattern, rest gathers every element after the named positions into a new array. It must be the last pattern item, as in const [first, ...others] = values.

const [first, ...others] = ['a', 'b', 'c'];

In an object pattern, rest gathers the properties not already selected into a new object. The original object is not changed, so const { id, ...details } = user leaves user intact.

const { id, ...details } = user;

Rest collects multiple values; spread expands an iterable or object.

const values = [1, 2];
const copy = [...values];
const [first, ...rest] = copy;

Yes. When no remaining arguments are supplied, the rest parameter is an empty array rather than undefined. That makes checks such as items.length === 0 straightforward.

function count(...items) {
  return items.length;
}
count();

They let a function accept a flexible number of arguments while keeping them easy to process.

function firstAndLast(first, ...middle) {
  return { first, middle };
}