Dot vs bracket notation and checking properties

Model answer. Dot notation (user.name) works when the key is a valid identifier written in code. Use bracket notation (user[key]) when:

• the key is in a variable or computed: scores[subject]
• the key has spaces, hyphens or starts with a digit: headers['content-type']
• the key is a symbol

Checking for a property:

const p = { name: 'Ravi', age: undefined };
'age' in p;                 // true (also true for inherited keys like 'toString')
Object.hasOwn(p, 'age');    // true, own keys only (modern, preferred)
p.age !== undefined;        // false! the key exists but holds undefined
p.address?.city;            // safe read of a possibly missing nested key

Use Object.hasOwn (or the older hasOwnProperty) when you need "own key exists", and in when inherited keys count too. Comparing with undefined cannot tell "missing" from "set to undefined".