JavaScript property enumerability

In most cases, object properties are enumerable by default, unless they are Symbols. This means that you can use the for...in loop to iterate over the properties of an object. Similarly, enumerable properties appear in object methods that enumerate the properties of an object. An example of this is the Object.keys() method, which will omit properties that are not enumerable. Finally, when using the object spread operator (...), only enumerable properties are copied to the new object.

const person = {
  name: 'John',
  surname: 'Doe',
  age: 30,
  socialSecurityNumber: '123-45-6789',
};

Object.defineProperty(person, 'socialSecurityNumber', {
  enumerable: false,
});

person.hasOwnProperty('socialSecurityNumber'); // true
person.propertyIsEnumerable('socialSecurityNumber'); // false

Object.keys(person); // ['name', 'surname', 'age']
Object.getOwnPropertyNames(person);
// ['name', 'surname', 'age', 'socialSecurityNumber']

const clone = { ...person };
clone.socialSecurityNumber; // undefined

To create a non-enumerable property, you can use Object.defineProperty() with the appropriate descriptor. You can check for the property's existence, using Object.prototype.hasOwnProperty() and for its enumerability, using Object.prototype.propertyIsEnumerable(). Additionally, in contrast to Symbols, non-enumerable properties will show up when using Object.getOwnPropertyNames().