JavaScript Optional Parameters: Defaults Apply to Undefined

The task

Build a retry description with defaults and call it with missing, undefined, null and zero values. Inspect the actual values before adding a validation policy.

Runnable Node 22 example

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

function describe(retries = 2, enabled = true) { return {retries, enabled}; }
assert.deepEqual(describe(), {retries: 2, enabled: true});
assert.deepEqual(describe(undefined, false), {retries: 2, enabled: false});
assert.deepEqual(describe(0, false), {retries: 0, enabled: false});
assert.deepEqual(describe(null, false), {retries: null, enabled: false});
assert.deepEqual(describe('', false), {retries: '', enabled: false});
console.log(JSON.stringify([describe(), describe(0, false), describe(null, false)]));

Expected output and explanation

The omitted call receives both defaults. Explicit undefined also triggers the first default, while false survives in the second position. Zero, null and empty text each remain the supplied value because none is undefined.

The function is intentionally an inspection example, not a validated retry API. A real retry function might reject null or noninteger text; that is a separate rule. Defaults can improve call convenience without making every accepted value valid.

Pitfall to check

Do not describe default parameters as “replace blanks” unless the contract actually checks blanks. Also avoid using truthy fallbacks inside the function to undo a caller’s deliberate zero after the parameter default has already been applied.