🔁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

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"

Previous"for-init" block Nextfor-of

Last updated 2 years ago

Was this helpful?