🔰property enumeration

• JS ⟩ value ⟩ object ⟩ property ⟩ enumerating properties

enumerate through properties of an object.

for (const key in obj) { ... }
for (const value of iterable) { ... }    // object is not iterable by default.

| ✅：string / symbol | 🔤：String | 🔺：Symbol

method/operator	own enum	own nonenum	inherited enum	inherited nonenum
Object.assign()	✅
...obj	✅
.keys(), .values(), .entries()	🔤
.getOwnPropertyNames()	🔤	🔤
.getOwnPropertySymbols()	🔺	🔺
.getOwnPropertyDescriptors()	✅	✅
.hasOwn()	✅	✅
Reflect.ownKeys()	✅	✅
for-in (loop over keys)	🔤		🔤
for-of (loop over values)	🔤		🔤

📘 MSN

⭐️

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

• String / Symbol

• enumerable / non-enumerable

• own / inherited (from the prototype chain)

📘 MSN

a property will not be enumerated if

• a property by that same name has already been enumerated

• a non-enumerable property by that same name has already been considered

🔴

• enumerable property.

• in operator

• for-in vs. for-of vs. in

👥

• property testing

• Proxy intercepts property enumeration.

• ...obj adds all enumerable own properties of an object to an object literal.

⬇️

• obj.mergeWith() - merge with other source objects.

📗

• JavaScript: The Definitive Guide ⟩ 6.6 Enumerating Properties

📘

• Enumerability and ownership of properties ⭐️

• Object.prototype ⟩

  • .hasOwnProperty()

  • .propertyIsEnumerable()

• Object ⟩

  • .hasOwn()

  • .getOwnPropertyNames()

• for...in

• in

• JavaScript Guide: Inheritance revisited