for...in

const object = { a: 1, b: 2, c: 3 };

for (const property in object) {
  console.log(`${property}: ${object[property]}`);
}

// Expected output:
// "a: 1"
// "b: 2"
// "c: 3"

for (variable in object)
  statement

const obj = { a: 1, b: 2, c: 3 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
}

// 
// "obj.a = 1"
// "obj.b = 2"
// "obj.c = 3"

const triangle = { a: 1, b: 2, c: 3 };

function ColoredTriangle() {
  this.color = "";
}

ColoredTriangle.prototype = triangle;

const obj = new ColoredTriangle();

for (const prop in obj) {
  if (Object.hasOwn(obj, prop)) {
    console.log(`obj.${prop} = ${obj[prop]}`);
  }
}

// 
// "obj.color = "

const obj = { a: 1, b: 2 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
  obj.c = 3;
}

// 
// obj.a = 1
// obj.b = 2

const proto = { a: 1 };
const obj = { __proto__: proto, a: 2 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
}

// 
// obj.a = 2

Object.defineProperty(obj, "a", { enumerable: false });

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
}
// a

const obj = { a: 1, b: 2 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
  Object.setPrototypeOf(obj, { c: 3 });
}

const obj = { a: 1, b: 2, c: 3 };

// 
for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
  delete obj.c;
}

const obj2 = { a: 1, b: 2, c: 3 };

// 
for (const prop in obj2) {
  console.log(`obj2.${prop} = ${obj2[prop]}`);
  delete obj2.a;
}

const proto = {};
const obj = { __proto__: proto, a: 1, b: 2 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
  proto.c = 3;
}

const obj = { a: 1, b: 2, c: 3 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
  Object.defineProperty(obj, "c", { enumerable: false });
}