descriptors.md

Property descriptors

An object property is actually a more complex and tunable thing than just a "key-value" mapping.

There are two kinds of properties.

The first is data properties.

They assiciate a key with the attributes:

• value -- the value of the property.
• writable -- if true, can be changed, otherwise it's read-only.
• enumerable -- if true, then listed in loops, otherwise not listed.
• configurable -- if true, the property can be deleted and these attributes can be modified, otherwise not.

All properties that we've seen yet were data properties.

By default when a property is created, all attributes are true.

We can get the the information about an existing property using Object.getOwnPropertyDescriptor:

let user = {
  name: "John"
};

let descriptor = Object.getOwnPropertyDescriptor(user, 'name');

alert( descriptor.value ); // John
alert( descriptor.writable ); // true
alert( descriptor.enumerable ); // true
alert( descriptor.configurable ); // true