Object.defineProperty()

const object = {};

Object.defineProperty(object, "foo", {
  value: 42,
  writable: false,
});

object.foo = 77;
// 

console.log(object.foo);
// : 42

Object.defineProperty(obj, prop, descriptor)

const obj = {};
// 1. null : 
const descriptor = Object.create(null);
descriptor.value = "static";

// 
Object.defineProperty(obj, "key", descriptor);

// 
Object.defineProperty(obj, "key2", {
  enumerable: false,
  configurable: false,
  writable: false,
  value: "static",
});

// 3. 
// (value, get, set, enumerable, writable, configurable)
Object.freeze(Object.prototype);

const o = {}; // 

// defineProperty 
// 
Object.defineProperty(o, "a", {
  value: 37,
  writable: true,
  enumerable: true,
  configurable: true,
});
// o  'a'  37 

// defineProperty 
// 
let bValue = 38;
Object.defineProperty(o, "b", {
  get() {
    return bValue;
  },
  set(newValue) {
    bValue = newValue;
  },
  enumerable: true,
  configurable: true,
});
o.b; // 38
// o  'b' 
//  38 
// o.b  bValue 

// : 
Object.defineProperty(o, "conflict", {
  value: 0x9f91102,
  get() {
    return 0xdeadbeef;
  },
});
// TypeError value 
// get 

const o = {}; // 

Object.defineProperty(o, "a", {
  value: 37,
  writable: false,
});

console.log(o.a); // 37 
o.a = 25; // 
// )
console.log(o.a); // 37 

// 
(() => {
  "use strict";
  const o = {};
  Object.defineProperty(o, "b", {
    value: 2,
    writable: false,
  });
  o.b = 3; // TypeError : "b" is read-only
  return o.b; //  2 (:)
})();

const o = {};
Object.defineProperty(o, "a", {
  value: 1,
  enumerable: true,
});
Object.defineProperty(o, "b", {
  value: 2,
  enumerable: false,
});
Object.defineProperty(o, "c", {
  value: 3,
}); // enumerable  false
o.d = 4; //  enumerable  true
Object.defineProperty(o, Symbol.for("e"), {
  value: 5,
  enumerable: true,
});
Object.defineProperty(o, Symbol.for("f"), {
  value: 6,
  enumerable: false,
});

for (const i in o) {
  console.log(i);
}
// 'a'  'd' 

Object.keys(o); // ['a', 'd']

o.propertyIsEnumerable("a"); // true
o.propertyIsEnumerable("b"); // false
o.propertyIsEnumerable("c"); // false
o.propertyIsEnumerable("d"); // true
o.propertyIsEnumerable(Symbol.for("e")); // true
o.propertyIsEnumerable(Symbol.for("f")); // false

const p = { ...o };
p.a; // 1
p.b; // undefined
p.c; // undefined
p.d; // 4
p[Symbol.for("e")]; // 5
p[Symbol.for("f")]; // undefined

const o = {};
Object.defineProperty(o, "a", {
  get() {
    return 1;
  },
  configurable: false,
});

Object.defineProperty(o, "a", {
  configurable: true,
}); // TypeError 
Object.defineProperty(o, "a", {
  enumerable: true,
}); // TypeError 
Object.defineProperty(o, "a", {
  set() {},
}); // TypeError  (set )
Object.defineProperty(o, "a", {
  get() {
    return 1;
  },
}); // TypeError 
// ( get )
Object.defineProperty(o, "a", {
  value: 12,
}); // TypeError 
// ('configurable'  false  'value'  'get' )

console.log(o.a); // logs 1
delete o.a; // 
console.log(o.a); // logs 1

const o = {};
Object.defineProperty(o, "b", {
  writable: true,
  configurable: false,
});
console.log(o.b); // undefined
Object.defineProperty(o, "b", {
  value: 1,
}); //  false 
console.log(o.b); // 1
o.b = 2; // 
console.log(o.b); // 2
// 
Object.defineProperty(o, "b", {
  writable: false,
});
Object.defineProperty(o, "b", {
  value: 1,
}); // TypeError: 
//  'b' 
// 

const o = {};
Object.defineProperty(o, "b", {
  writable: false,
  configurable: true,
});
Object.defineProperty(o, "b", {
  value: 1,
}); // defineProperty 
console.log(o.b); // 1
o.b = 2; //  TypeError : 

const o = {};
Object.defineProperty(o, "b", {
  writable: false,
  configurable: false,
});
Object.defineProperty(o, "b", {
  value: 1,
}); // TypeError: 

const o = {};

o.a = 1;
// 
Object.defineProperty(o, "a", {
  value: 1,
  writable: true,
  configurable: true,
  enumerable: true,
});

// 
Object.defineProperty(o, "a", { value: 1 });
// 
Object.defineProperty(o, "a", {
  value: 1,
  writable: false,
  configurable: false,
  enumerable: false,
});

function Archiver() {
  let temperature = null;
  const archive = [];

  Object.defineProperty(this, "temperature", {
    get() {
      console.log("get!");
      return temperature;
    },
    set(value) {
      temperature = value;
      archive.push({ val: temperature });
    },
  });

  this.getArchive = () => archive;
}

const arc = new Archiver();
arc.temperature; // 'get!'
arc.temperature = 11;
arc.temperature = 13;
arc.getArchive(); // [{ val: 11 }, { val: 13 }]

const pattern = {
  get() {
    return "";
  },
  set() {
    this.myName = "";
  },
};

function TestDefineSetAndGet() {
  Object.defineProperty(this, "myProperty", pattern);
}

const instance = new TestDefineSetAndGet();
instance.myProperty = "test";
console.log(instance.myProperty);
// 

console.log(instance.myName); // 

function MyClass() {}

let value;
Object.defineProperty(MyClass.prototype, "x", {
  get() {
    return value;
  },
  set(x) {
    value = x;
  },
});

const a = new MyClass();
const b = new MyClass();
a.x = 1;
console.log(b.x); // 1

function MyClass() {}

Object.defineProperty(MyClass.prototype, "x", {
  get() {
    return this.storedX;
  },
  set(x) {
    this.storedX = x;
  },
});

const a = new MyClass();
const b = new MyClass();
a.x = 1;
console.log(b.x); // undefined

function MyClass() {}

MyClass.prototype.x = 1;
Object.defineProperty(MyClass.prototype, "y", {
  writable: false,
  value: 1,
});

const a = new MyClass();
a.x = 2;
console.log(a.x); // 2
console.log(MyClass.prototype.x); // 1
a.y = 2; // 
console.log(a.y); // 1
console.log(MyClass.prototype.y); // 1