Lære Property Descriptors | Understanding and Working with Objects

Property descriptors in JavaScript provide fine-grained control over the behavior of individual object properties. Each property on an object has associated attributes, known as property descriptors, that determine how the property can be used. The four main descriptors are configurable, enumerable, writable, and value.

configurable: determines whether the property can be deleted or its descriptors can be changed;
enumerable: controls whether the property appears during enumeration of the object's properties, such as in a for...in loop;
writable: specifies if the value of the property can be changed;
value: holds the actual data stored in the property.

Understanding these descriptors allows you to define and manage object properties with precision, enabling patterns such as immutability, hidden properties, and more.


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const user = {};

Object.defineProperty(user, "id", {
  value: 101,
  writable: false,
  enumerable: false,
  configurable: false
});

// Try to change the value
user.id = 202;

// Try to enumerate properties
for (let key in user) {
  console.log(key); // "id" will not be logged
}

// Try to delete the property
delete user.id; // returns false

console.log(user.id); // 101

In this example, you use Object.defineProperty to add an id property to the user object. The property descriptors set for id have important effects:

Setting writable to false means attempts to change user.id will be ignored (or throw an error in strict mode), so the value remains 101;
Setting enumerable to false hides the property from enumeration, so id does not show up in for...in loops or Object.keys;
Setting configurable to false prevents the property from being deleted or its descriptors from being changed;
The value is set to 101, which is the actual data stored in the property.

These settings create a property that is read-only, hidden from enumeration, and cannot be deleted or reconfigured.


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const user = {};

Object.defineProperty(user, "id", {
  value: 101,
  writable: false,
  enumerable: false,
  configurable: false
});

const descriptor = Object.getOwnPropertyDescriptor(user, "id");
console.log(JSON.stringify(descriptor));

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configurable: determines whether the property can be deleted or its descriptors can be changed;
enumerable: controls whether the property appears during enumeration of the object's properties, such as in a for...in loop;
writable: specifies if the value of the property can be changed;
value: holds the actual data stored in the property.