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1-js/9-object-inheritance/02-property-accessors/article.md

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+# Property getters and setters [todo move to objects?]
+
+There are two kinds of properties.
+
+The first kind is *data properties*. We already know how to work with them, actually, all properties that we've been using yet are data properties.
+
+The second type of properties is something new. It's *accessor properties*. They are essentially functions that work on getting and setting a value, but look like regular properties.
+
+[cut]
+
+## Getters and setters
+
+Accessor properties are represented by "getter" and "setter" methods. In an object literal they are preprended with words `get` and `set`:
+
+```js
+let obj = {
+  *!*get propName()*/!* {
+    // getter, the code executed on getting obj.propName
+  },
+
+  *!*set propName(value)*/!* {
+    // setter, the code executed on setting obj.propName = value
+  }
+};
+```
+
+The getter works when `obj.propName` is read, the setter -- when it is assigned.
+
+For instance, we have a `user` object with `name` and `surname`:
+
+```js run
+let user = {
+  name: "John",
+  surname: "Smith"
+};
+```
+
+Now we want to add a "fullName" property, that should be "John Smith". Of course, we don't want to copy-paste existing information, so we can implement it as an accessor:
+
+```js run
+let user = {
+  name: "John",
+  surname: "Smith",
+
+*!*
+  get fullName() {
+    return `${this.name} ${this.surname}`;
+  }
+*/!* 
+};
+
+*!*
+alert(user.fullName); // John Smith
+*/!*
+```
+
+From outside, an accessor property looks like a regular one. That's the idea of accessor properties. We don't call `user.fullName` as a function, we read it normally, and it runs behind the scenes.
+
+As of now, `fullName` has only a getter. If we attempt to assign `user.fullName=`, there will be an error. 
+
+Let's fix it by adding a setter for `user.fullName`:
+
+```js run
+let user = {
+  name: "John",
+  surname: "Smith",
+
+  get fullName() {
+    return `${this.name} ${this.surname}`;
+  },
+
+*!*
+  set fullName(value) {
+    [this.name, this.surname] = value.split(" ");
+  }
+*/!* 
+};
+
+// set fullName is executed with the given value.
+user.fullName = "Alice Cooper";
+
+alert(user.name); // Alice
+alert(user.surname); // Cooper
+```
+
+Now we have a "virtual" property. It is readable and writable, but in fact does not exist. 
+
+```smart
+We can either work with a property as a "data property" or as an "accessor property", these two never mix.
+
+Once a property as defined with `get prop()`, it can't be assigned with `obj.prop=`, unless there's a setter too.
+
+And if a property is defined with `set prop()`, then it can't be read unless there's also a getter.
+```
+
+
+## Accessor descriptors
+
+Differences between data properties and accessors are also reflected in their descriptors.
+
+For accessor properties, there is no `value` and `writable`, but instead there are `get` and `set` functions.
+
+So an accessor descriptor may have:
+
+- **`get`** -- a function without arguments, that works when a property is read,
+- **`set`** -- a function with one argument, that is called when the property is set,
+- **`enumerable`** -- same as for data properties,
+- **`configurable`** -- same as for data properties.
+
+For instance, to create an accessor `fullName` with `defineProperty`, we can pass a descriptor with `get` and `set`:
+
+```js run
+let user = {
+  name: "John",
+  surname: "Smith"
+};
+
+*!*
+Object.defineProperty(user, 'fullName', {
+  get() {
+    return `${this.name} ${this.surname}`;
+  },
+
+  set(value) {
+    [this.name, this.surname] = value.split(" ");
+  }
+*/!* 
+});
+
+alert(user.fullName); // John Smith
+
+for(let key in user) alert(key); 
+```
+
+## Smarter getters/setters
+
+A combination of getter/setter can be used to validate property values at the moment of assignment.
+
+For instance, if we want to forbid too short names for `user`, we can store `name` in a special property `_name`, at the same time providing smart getter/setter for it:
+
+```js run
+let user = {
+  get name() {
+    return this._name;
+  },
+
+  set name(value) {
+    if (value.length < 4) {
+      throw new Error("Name is too short, need at least 4 characters");
+    }
+    this._name = value;
+  }
+};
+
+user.name = "Pete"; 
+alert(user.name); // Pete
+
+user.name = ""; // Error
+```
+
+Technically, the "real" name is stored in `user._name`, so the outer code may access it. In Javascript there's no way to prevent reading an object property. But there is a widely known agreement that properties starting with an underscore `"_"` are internal and should not be touched from outside.
+
+
+## For compatibility
+
+One of great ideas behind getters and setters -- they allow to take control over a property at any moment.
+
+For instance, we start implementing user objects using data properties `name` and `age`:
+
+```js
+function User(name, age) {
+  this.name = name;
+  this.age = age;
+}
+
+let john = new User("John", 25);
+
+alert( john.age ); // 25
+```
+
+...But sooner or later, things may change. Instead of `age` we may decide to store `birthday`, because it's more precise and convenient:
+
+```js
+function User(name, birthday) {
+  this.name = name;
+  this.birthday = birthday;
+}
+
+let john = new User("John", new Date(1992, 6, 1));
+```
+
+Now what to do with the old code that still uses `age`?
+
+We can try to find all such places and fix them, but that takes time and not always possible with 3rd party libraries. And besides, `age` is a nice thing to have in `user`, right? In some places it's just what we want.
+
+Adding a getter for `age` mitigates the problem:
+
+```js run no-beautify
+function User(name, birthday) {
+  this.name = name;
+  this.birthday = birthday;
+
+*!*
+  // age is calculated from the current date and birthday
+  Object.defineProperty(this, "age", {
+    get() {
+      let todayYear = new Date().getFullYear();
+      return todayYear - this.birthday.getFullYear();
+    }
+  });
+*/!*
+}
+
+let john = new User("John", new Date(1992, 6, 1));
+
+alert( john.birthday ); // birthday is available
+alert( john.age );      // ...as well as the age
+```
+
+