JavaScript Null Check: Distinguish Null, Undefined and a Value

The task

Classify four small records without substituting a fallback. The function reports state; it does not invent a name or make a required-field decision.

Runnable Node 22 example

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

function state(record) {
  if (!Object.hasOwn(record, 'middle')) return 'absent';
  if (record.middle === null) return 'explicitly empty';
  if (record.middle === undefined) return 'undefined';
  return 'provided';
}
assert.equal(state({}), 'absent');
assert.equal(state({middle: null}), 'explicitly empty');
assert.equal(state({middle: undefined}), 'undefined');
assert.equal(state({middle: ''}), 'provided');
console.log([{}, {middle: null}, {middle: undefined}, {middle: 'Lee'}].map(state).join(' | '));

Expected output and explanation

The own-property check separates absence from an explicitly present undefined field. Strict equality then identifies null without collapsing it into another state. An empty string reaches provided because this function’s stated purpose is state classification, not name validation.

That distinction prevents a common overclaim: a present field is not necessarily a valid value. A later validator could reject empty text, limit accepted types or require another field. Keep those rules visible rather than making the null test silently do everything.

Pitfall to check

A loose value == null check groups null and undefined. That can be intentional when both mean the same thing, but it would discard a distinction this contract requires. Use ?? for a separate fallback task only after deciding which states may be replaced.