What are property descriptors and property flags (writable, enumerable, configurable)?

Every object property has a descriptor with 3 flags:

• writable — can the value be changed?
• enumerable — does it show up in for...in / Object.keys()?
• configurable — can the descriptor be changed? Can the property be deleted?

const obj = {};

Object.defineProperty(obj, 'ID', {
  value: 42,
  writable: false,    // read-only
  enumerable: false,  // hidden from loops
  configurable: false // can't redefine or delete
});

obj.ID = 99;              // silently fails (TypeError in strict)
console.log(obj.ID);      // 42

Object.keys(obj);         // [] — ID is non-enumerable
delete obj.ID;            // false — non-configurable

// View a property's descriptor
Object.getOwnPropertyDescriptor(obj, 'ID');
// { value: 42, writable: false, enumerable: false, configurable: false }

// Regular property defaults:
// { value: ..., writable: true, enumerable: true, configurable: true }