🔁for-in

loop over "keys"

JS⟩ syntax ⟩ for loops ⟩ for-in

iterates over all (own/inherited) enumerable string properties of an object (ignoring properties keyed by symbols).

for (const <key> in obj) {}    // loop over "keys"
for (<variable> in obj) {}     
// <variable> can be a let/const/var, or an "lvalue" (assignment target). 

⭐️

const

• can be used with for-in / for-of loop.

• can't be used with a classic for-loop.

👉 no const for classic "for"❗

⭐️ 注意： for...in, for...of 有 "(...)" 括號❗️

for (let key   in object) {}    // loop over "keys"
for (let value of iterable) {}    // loop over "values"
//  ^                     ^       

👉 Property flags and descriptors - “flags-aware” clone

🗺️

✅：string / symbol | ❌：not available | 🔤：String | 🔺：Symbol

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

👉 property enumeration (📘 MSN )

👥

• iterable

• for-of

• "pure" object is an object without prototype.
• arr.entries

• lvalue can be used in for-in/for-of.

• parentheses () are used in for-in/for-of.

💈

let animal = {
  eats: true
};

let rabbit = {
  jumps: true,
  __proto__: animal
};

// own keys
Object.keys(rabbit)               // "jumps"

// inherited included
for(let prop in rabbit) log(prop) // "jumps", "eats"

📗

• JS.info ⟩ prototypal inheritance ⟩ for...in iterates over (enumerable) properties (inherited properties included)
• JavaScript: The Definitive Guide ⟩ 5.4.5 for/in

• 談談 JavaScript 中 for ... in
• for...in 與 for...of 的差別
• JavaScript 之旅 (4)：Object.keys() & Object.values() & Object.entries() ⭐️
• JavaScript traits: the clean way to modify global prototypes

📘

• for...in ⟩

  • Deleted, added, or modified properties ⭐️
• Object ⟩

  • Object.keys()
  • Object.values()
  • Object.entries()
• Object.prototype ⟩

  • .hasOwnProperty()
• "in" operator