JavaScript Function Arguments: Parameters, Rest and Extra Values

The task

Inspect a report function with one named parameter and a rest array. Compare an omitted argument with an explicit undefined value using the actual call count.

Runnable Node 22 example

const assert = require('node:assert/strict');

function inspect(title, ...extra) {
  return {title, extra, supplied: arguments.length};
}
assert.deepEqual(inspect('weekly', 4, true), {title: 'weekly', extra: [4, true], supplied: 3});
assert.deepEqual(inspect(), {title: undefined, extra: [], supplied: 0});
assert.deepEqual(inspect(undefined), {title: undefined, extra: [], supplied: 1});
console.log(JSON.stringify(inspect('weekly', 4, true)));

Expected output and explanation

The named title parameter receives the first supplied value. Rest collects the remaining values into an array, so four and true remain separate elements rather than being silently discarded. arguments.length records how many values the ordinary function call supplied.

The empty call and explicit undefined call produce the same title value but different counts. That distinction can matter in a deliberately defined API, although most functions should prefer a simpler contract. Default-parameter behavior is handled in the linked concept rather than inferred from this inspection.

Pitfall to check

Arrow functions do not provide their own arguments object. Use rest when you want the arrow’s supplied values. Also do not expose or log arbitrary extra arguments in a real application when they may contain confidential input.