🔰property testing

• JS ⟩ value ⟩ object ⟩ property ⟩ testing properties

check whether an object has a property.

'prop' in obj                     // in operator
Object.hasOwn(obj, 'prop')        // recommended
obj.hasOwnProperty('prop')        // not recommended
obj.propertyIsEnumerable('prop')

⭐ the following methods/operators all support String / Symbol keys.

method/operator	own enum	own nonenum	inherited enum	inherited nonenum
in	✅	✅	✅	✅
.hasOwn()	✅	✅
.hasOwnProperty()	✅	✅
.propertyIsEnumerable()	✅

⭐️

Every property in JavaScript objects can be classified by 3 factors:

• String / Symbol

• enumerable / non-enumerable

• own / inherited (from the prototype chain)

📘 MSN

🔴

• in operator

• for-in vs. for-of vs. in

👥

• property enumeration

• Proxy intercepts property testing.

📗

• JavaScript: The Definitive Guide ⟩ 6.5 Testing Properties

📘

• Enumerability and ownership of properties ⭐️

• Object.prototype ⟩

  • .hasOwnProperty()

  • .propertyIsEnumerable()

• Object ⟩

  • .hasOwn()

  • .getOwnPropertyNames()

• for...in

• in

• JavaScript Guide: Inheritance revisited