typeof in JavaScript: Inspect Values and Handle Null and Arrays

The task

Classify a short mixed-value list and then explicitly identify its array and null entries. The example avoids treating every object-like result as an editable record.

Runnable Node 22 example

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

const values = [17, '17', false, undefined, null, [], () => 1];
const types = values.map(value => typeof value);
assert.deepEqual(types, ['number', 'string', 'boolean', 'undefined', 'object', 'object', 'function']);
assert.equal(values[4] === null, true);
assert.equal(Array.isArray(values[5]), true);
assert.equal(typeof NaN, 'number');
console.log(types.join(','));

Expected output and explanation

The numeric-looking string remains a string; typeof does not convert it. The null and array entries share the word object but differ under the explicit tests. A function has its own typeof result, while NaN still belongs to the number type.

Choose the next check from the contract. Array.isArray answers whether a value is an array; equality with null answers whether that particular empty marker is present. Neither establishes all the fields or invariants of a business record. An input validator may need several checks rather than one typeof expression.

Pitfall to check

An object result does not make property access safe on null. Guard it before reading fields. Also distinguish an unknown identifier from a known property whose value is undefined when writing diagnostics; the dedicated undefined check covers that boundary.