Property flags and descriptors

As we know, objects can store properties.

Till now, a property was a simple “key-value” pair to us. But an object property is actually more complex and tunable thing.

Property flags

Object properties, besides a value, have three special attributes (so-called “flags”):

  • 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.

We didn’t see them yet, because generally they do not show up. When we create a property “the usual way”, all of them are true. But we also can change them any time.

First, let’s see how to get those flags.

The method Object.getOwnPropertyDescriptor allows to query the full information about a property.

The syntax is:

let descriptor = Object.getOwnPropertyDescriptor(obj, propertyName);
obj
The object to get information from.
propertyName
The name of the property.

The returned value is a so-called “property descriptor” object: it contains the value and all the flags.

For instance:

let user = {
  name: "John"
};

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

alert( JSON.stringify(descriptor, null, 2 ) );
/* property descriptor:
{
  "value": "John",
  "writable": true,
  "enumerable": true,
  "configurable": true
}
*/

To change the flags, we can use Object.defineProperty.

The syntax is:

Object.defineProperty(obj, propertyName, descriptor)
obj, propertyName
The object and property to work on.
descriptor
Property descriptor to apply.

If the property exists, defineProperty updates its flags. Otherwise, it creates the property with the given value and flags; in that case, if a flag is not supplied, it is assumed false.

For instance, here a property name is created with all falsy flags:

let user = {};

Object.defineProperty(user, "name", {
  value: "John"
});

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

alert( JSON.stringify(descriptor, null, 2 ) );
/*
{
  "value": "John",
  "writable": false,
  "enumerable": false,
  "configurable": false
}
 */

Compare it with “normally created” user.name above: now all flags are falsy. If that’s not what we want then we’d better set them to true in descriptor.

Now let’s see effects of the flags by example.

Read-only

Let’s make user.name read-only by changing writable flag:

let user = {
  name: "John"
};

Object.defineProperty(user, "name", {
  writable: false
});

user.name = "Pete"; // Error: Cannot assign to read only property 'name'...

Now no one can change the name of our user, unless he applies his own defineProperty to override ours.

Here’s the same operation, but for the case when a property doesn’t exist:

let user = { };

Object.defineProperty(user, "name", {
  value: "Pete",
  // for new properties need to explicitly list what's true
  enumerable: true,
  configurable: true
});

alert(user.name); // Pete
user.name = "Alice"; // Error

Non-enumerable

Now let’s add a custom toString to user.

Normally, a built-in toString for objects is non-enumerable, it does not show up in for..in. But if we add toString of our own, then by default it shows up in for..in, like this:

let user = {
  name: "John",
  toString() {
    return this.name;
  }
};

// By default, both our properties are listed:
for(let key in user) alert(key); // name, toString

If we don’t like it, then we can set enumerable:false. Then it won’t appear in for..in loop, just like the built-in one:

let user = {
  name: "John",
  toString() {
    return this.name;
  }
};

Object.defineProperty(user, "toString", {
  enumerable: false
});

// Now our toString disappears:
for(let key in user) alert(key); // name

Non-enumerable properties are also excluded from Object.keys:

alert(Object.keys(user)); // name

Non-configurable

The non-configurable flag (configurable:false) is sometimes preset for built-in objects and properties.

A non-configurable property can not be deleted or altered with defineProperty.

For instance, Math.PI is both read-only, non-enumerable and non-configurable:

let descriptor = Object.getOwnPropertyDescriptor(Math, 'PI');

alert( JSON.stringify(descriptor, null, 2 ) );
/*
{
  "value": 3.141592653589793,
  "writable": false,
  "enumerable": false,
  "configurable": false
}
*/

So, a programmer is unable to change the value of Math.PI or overwrite it.

Math.PI = 3; // Error

// delete Math.PI won't work either

Making a property non-configurable is a one-way road. We cannot change it back, because defineProperty doesn’t work on non-configurable properties.

Here we are making user.name a “forever sealed” constant:

let user = { };

Object.defineProperty(user, "name", {
  value: "John",
  writable: false,
  configurable: false
});

// won't be able to change user.name or its flags
// all this won't work:
//   user.name = "Pete"
//   delete user.name
//   defineProperty(user, "name", ...)
Object.defineProperty(user, "name", {writable: true}); // Error
Errors appear only in use strict

In the non-strict mode, no errors occur when writing to read-only properties and such. But the operation still won’t succeed. Flag-violating actions are just silently ignored in non-strict.

Object.defineProperties

There’s a method Object.defineProperties(obj, descriptors) that allows to define many properties at once.

The syntax is:

Object.defineProperties(obj, {
  prop1: descriptor1,
  prop2: descriptor2
  // ...
});

For instance:

Object.defineProperties(user, {
  name: { value: "John", writable: false },
  surname: { value: "Smith", writable: false },
  // ...
});

So, we can set many properties at once.

Object.getOwnPropertyDescriptors

To get many descriptors at once, we can use the method Object.getOwnPropertyDescriptors(obj).

Together with Object.defineProperties it can be used as a “flags-aware” way of cloning an object:

let clone = Object.defineProperties({}, Object.getOwnPropertyDescriptors(obj));

Normally when we clone an object, we use an assignment to copy properties, like this:

for(let key in user) {
  clone[key] = user[key]
}

…But that does not copy flags. So if we want a “better” clone then Object.defineProperties is preferred.

Sealing an object globally

Property descriptors work at the level of individual properties.

There are also methods that limit access to the whole object:

Object.preventExtensions(obj)
Forbids to add properties to the object.
Object.seal(obj)
Forbids to add/remove properties, sets for all existing properties configurable: false.
Object.freeze(obj)
Forbids to add/remove/change properties, sets for all existing properties configurable: false, writable: false.

And also there are tests for them:

Object.isExtensible(obj)
Returns false if adding properties is forbidden, otherwise true.
Object.isSealed(obj)
Returns true if adding/removing properties is forbidden, and all existing properties have configurable: false.
Object.isFrozen(obj)
Returns true if adding/removing/changing properties is forbidden, and all current properties are configurable: false, writable: false.

These methods are rarely used in practice.

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