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1-js/9-object-inheritance/01-property-flags-descriptors/article.md

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+
+# Property flags and descriptors [todo move to objects?]
+
+Now as we know how to work with primitives and several concrete object-based types, let's return to objects in general.
+
+As we know, objects can store properties.
+
+But an object property is actually more complex thing than just a "key-value" mapping.
+
+[cut]
+
+## 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 by default they are concealed. 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 read the flags. 
+
+The method [Object.getOwnPropertyDescriptor](mdn:js/Object/getOwnPropertyDescriptor) allows to query the information about a property.
+
+The syntax is:
+```js
+let descriptor = Object.getOwnPropertyDescriptor(obj, propertyName);
+```
+
+`obj`
+: The object to get information about.
+
+`propertyName`
+: The name of the property of interest.
+
+The returned value is a so-called "property descriptor" object: it contains the value and all the flags.
+
+For instance:
+
+```js run
+let user = {
+  name: "John"
+};
+
+let descriptor = Object.getOwnPropertyDescriptor(user, 'name');
+
+alert( JSON.stringify(descriptor, null, 2 ) );
+/* descriptor:
+{
+  "value": "John",
+  "writable": true,
+  "enumerable": true,
+  "configurable": true
+}
+ */
+```
+
+Now, we'll use [Object.defineProperty](mdn:js/Object/defineProperty) to change flags.
+
+The syntax is:
+
+```js
+Object.defineProperty(obj, propertyName, descriptor)
+```
+
+`obj`, `propertyName`
+: The object and property to work on.
+
+`descriptor`
+: Property descriptor to apply.
+
+If the property exist, it update its flags.
+
+Otherwise, it creates the property with the provided value and flags. Please note, that if a flag is not supplied, it is assumed `false`.
+
+For instance, here a property `name` is created with all falsy flags:
+
+```js run
+let user = {};
+
+Object.defineProperty(user, "name", {
+  value: "John"
+});
+
+let descriptor = Object.getOwnPropertyDescriptor(user, 'name');
+
+alert( JSON.stringify(descriptor, null, 2 ) );
+/* compare it with "normally created" user.name above:
+{
+  "value": "John",
+*!*
+  "writable": false,
+  "enumerable": false,
+  "configurable": false
+*/!*
+}
+ */
+``` 
+
+Now let's see effects of the flags by example.
+
+## Read-only
+
+Let's make `user.name` read-only by changing `writable` flag:
+
+```js run
+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:
+
+```js run
+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 a custom `toString` to `user`. 
+
+Normally, a built-in `toString` for objects is non-enumerable, it does not show up in `for..in`. So we'll make ours behave the same.
+
+```js run
+let user = {
+  name: "John",
+  toString() {
+    return this.name;
+  }
+};
+
+// By default, both our properties are listed:
+for(let key in user) alert(key); // name, toString
+
+Object.defineProperty(user, "toString", {
+*!*
+  enumerable: false
+*/!*
+});
+
+*!*
+// Now toString disappears:
+*/!*
+for(let key in user) alert(key); // name
+```
+
+Non-enumerable properties are also excluded from `Object.keys`. 
+
+## Non-configurable
+
+Non-configurable flag is often 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:
+
+```js run
+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 that built-in constant or overwrite it.
+
+```js run
+Math.PI = 3; // Error
+
+// delete Math.PI won't work either
+```
+
+Making non-configurable is one-way road. We cannot change it back, because `defineProperty` doesn't work on non-configurable properties.
+
+Here `user.name` is a forever sealed constant:
+
+```js run
+let user = { };
+
+Object.defineProperty(user, "name", {
+  value: "John",
+  writable: false,
+  configurable: false
+});
+
+*!*
+// can't change it or its flags
+// user.name = "Pete" won't work
+// delete user.name won't work
+// defineProperty won't work either:
+Object.defineProperty(user, "name", {writable: true}); // Error
+*/!*
+```
+
+```smart header="Errors appear only in use strict"
+In non-strict mode, there are no errors for writing to read-only properties and such, flag-violating actions are silently ignored.
+```
+
+## Many properties at once
+
+There's a method [Object.defineProperties(obj, descriptors)](mdn:js/Object/defineProperties) that allows to define many properties at once:
+
+```js
+Object.defineProperties(user, {
+  name: { writable: false },
+  surname: { ... },
+  // ...
+});
+```
+
+And, to get all descriptors, use [Object.getOwnPropertyDescriptors(obj)](mdn:js/Object/getOwnPropertyDescriptors).
+
+Together they can be used as an "property flags-aware" way of cloning an object:
+
+```js
+let clone = Object.defineProperties({}, Object.getOwnPropertyDescriptors(obj));
+```
+
+(For the clone to be fully identical, we need to care about one more thing: "prototype", we'll see into it soon in the chapter [todo])
+
+## Sealing an object globally
+
+Property descriptors allow to forbid modifications of individual properties.
+
+There are also methods that limit access to the whole object:
+
+[Object.preventExtensions(obj)](mdn:js/Object/preventExtensions)
+: Forbids to add properties to the object.
+
+[Object.seal(obj)](mdn:js/Object/seal)
+: Forbids to add/remove properties, sets for all existing properties `configurable: false`.
+
+[Object.freeze(obj)](mdn:js/Object/freeze)
+: Forbids to add/remove/change properties, sets for all existing properties `configurable: false, writable: false`.
+
+And the tests for them:
+
+[Object.isExtensible(obj)](mdn:js/Object/isExtensible)
+: Returns `false` if adding properties is forbidden, otherwise `true`.
+
+[Object.isSealed(obj)](mdn:js/Object/isSealed)
+: Returns `true` if adding/removing properties is forbidden, and all existing properties have `configurable: false`.
+
+[Object.isFrozen(obj)](mdn:js/Object/isFrozen)
+: 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.
+
+
+
+
+## Tasks
+
+Check: new property has all non-enum, test