🔁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

🗺️

👥

💈

📗

📘