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Original file line number | Diff line number | Diff line change | ||||
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# Property getters and setters | ||||||
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There are two kinds of properties. | ||||||
Hay dos tipos de propiedades de objetos. | ||||||
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The first kind is *data properties*. We already know how to work with them. All properties that we've been using until now were data properties. | ||||||
El primer tipo son las *propiedades de los datos*. Ya sabemos cómo trabajar con ellas. En realidad, todas las propiedades que hemos estado usando hasta ahora eran propiedades de datos. | ||||||
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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 to an external code. | ||||||
El segundo tipo de propiedades es algo nuevo. Son las *propiedades de acceso*. Estas son esencialmente funciones que se ejecutan para la obtención y asignación de un valor, pero parecen propiedades normales para un código externo. | ||||||
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## Getters and setters | ||||||
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Accessor properties are represented by "getter" and "setter" methods. In an object literal they are denoted by `get` and `set`: | ||||||
Las propiedades de acceso están representadas por métodos "getter" y "setter". Propiamente, en un objeto se denotan por `get` y `set`: | ||||||
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```js | ||||||
let obj = { | ||||||
*!*get propName()*/!* { | ||||||
// getter, the code executed on getting obj.propName | ||||||
// getter, el código ejecutado para obtener obj.propName | ||||||
}, | ||||||
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*!*set propName(value)*/!* { | ||||||
// setter, the code executed on setting obj.propName = value | ||||||
// setter, el código ejecutado para asignar obj.propName = value | ||||||
} | ||||||
}; | ||||||
``` | ||||||
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The getter works when `obj.propName` is read, the setter -- when it is assigned. | ||||||
El getter funciona cuando se lee `obj.propName`, el setter -- cuando se asigna. | ||||||
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For instance, we have a `user` object with `name` and `surname`: | ||||||
Por ejemplo, tenemos un objeto "usuario" con "nombre" y "apellido": | ||||||
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```js | ||||||
let user = { | ||||||
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@@ -34,7 +34,7 @@ let user = { | |||||
}; | ||||||
``` | ||||||
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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: | ||||||
Ahora queremos añadir una propiedad de "Nombre completo" (`fullName`), que debería ser `"John Smith"`. Por supuesto, no queremos copiar-pegar la información existente, así que podemos aplicarla como una propiedad de acceso: | ||||||
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```js run | ||||||
let user = { | ||||||
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@@ -53,9 +53,9 @@ alert(user.fullName); // John Smith | |||||
*/!* | ||||||
``` | ||||||
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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: the getter runs behind the scenes. | ||||||
Desde fuera, una propiedad de acceso se parece a una normal. Esa es la idea de estas propiedades. No *llamamos* a `user.fullName` como una función, la *leemos* normalmente: el "getter" corre detrás de la escena. | ||||||
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As of now, `fullName` has only a getter. If we attempt to assign `user.fullName=`, there will be an error: | ||||||
A partir de ahora, "Nombre completo" sólo tiene un receptor. Si intentamos asignar `user.fullName=`, habrá un error. | ||||||
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```js run | ||||||
let user = { | ||||||
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@@ -69,7 +69,7 @@ user.fullName = "Test"; // Error (property has only a getter) | |||||
*/!* | ||||||
``` | ||||||
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Let's fix it by adding a setter for `user.fullName`: | ||||||
Arreglémoslo agregando un setter para `user.fullName`: | ||||||
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```js run | ||||||
let user = { | ||||||
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@@ -87,29 +87,30 @@ let user = { | |||||
*/!* | ||||||
}; | ||||||
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// set fullName is executed with the given value. | ||||||
// set fullName se ejecuta con el valor dado. | ||||||
user.fullName = "Alice Cooper"; | ||||||
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alert(user.name); // Alice | ||||||
alert(user.surname); // Cooper | ||||||
``` | ||||||
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As the result, we have a "virtual" property `fullName`. It is readable and writable. | ||||||
Como resultado, tenemos una propiedad virtual `fullName` que puede leerse y escribirse. | ||||||
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## Accessor descriptors | ||||||
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Descriptors for accessor properties are different from those for data properties. | ||||||
Los descriptores de las propiedades de acceso son diferentes de aquellos para las propiedades de los datos. | ||||||
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For accessor properties, there is no `value` or `writable`, but instead there are `get` and `set` functions. | ||||||
Para las propiedades de acceso, no hay cosas como "valor" y "escritura", sino de "get" y "set". | ||||||
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That is, an accessor descriptor may have: | ||||||
Así que un descriptor de accesos puede tener: | ||||||
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- **`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. | ||||||
- **`get`** -- una función sin argumentos, que funciona cuando se lee una propiedad, | ||||||
- **`set`** -- una función con un argumento, que se llama cuando se establece la propiedad, | ||||||
- **`enumerable`** -- lo mismo que para las propiedades de los datos, | ||||||
- **`configurable`** -- lo mismo que para las propiedades de los datos. | ||||||
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For instance, to create an accessor `fullName` with `defineProperty`, we can pass a descriptor with `get` and `set`: | ||||||
Por ejemplo, para crear un acceso " Nombre Completo" con "Definir Propiedad", podemos pasar un descriptor con `get` y `set`: | ||||||
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Suggested change
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```js run | ||||||
let user = { | ||||||
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@@ -134,13 +135,13 @@ alert(user.fullName); // John Smith | |||||
for(let key in user) alert(key); // name, surname | ||||||
``` | ||||||
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Please note that a property can be either an accessor (has `get/set` methods) or a data property (has a `value`), not both. | ||||||
Tenga en cuenta que una propiedad puede ser un acceso (tiene métodos `get/set`) o una propiedad de datos (tiene un 'valor'), no ambas. | ||||||
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If we try to supply both `get` and `value` in the same descriptor, there will be an error: | ||||||
Si intentamos poner tanto `get` como `valor` en el mismo descriptor, habrá un error: | ||||||
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```js run | ||||||
*!* | ||||||
// Error: Invalid property descriptor. | ||||||
// Error: Descriptor de propiedad inválido. | ||||||
*/!* | ||||||
Object.defineProperty({}, 'prop', { | ||||||
get() { | ||||||
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@@ -153,9 +154,9 @@ Object.defineProperty({}, 'prop', { | |||||
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## Smarter getters/setters | ||||||
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Getters/setters can be used as wrappers over "real" property values to gain more control over operations with them. | ||||||
Getters/setters pueden ser usados como envoltorios sobre valores de propiedad "reales" para obtener más control sobre ellos. | ||||||
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For instance, if we want to forbid too short names for `user`, we can have a setter `name` and keep the value in a separate property `_name`: | ||||||
Por ejemplo, si queremos prohibir nombres demasiado cortos para "usuario", podemos guardar "nombre" en una propiedad especial "nombre". Y filtrar las asignaciones en el setter: | ||||||
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```js run | ||||||
let user = { | ||||||
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@@ -165,7 +166,7 @@ let user = { | |||||
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set name(value) { | ||||||
if (value.length < 4) { | ||||||
alert("Name is too short, need at least 4 characters"); | ||||||
alert("El nombre es demasiado corto, necesita al menos 4 caracteres"); | ||||||
return; | ||||||
} | ||||||
this._name = value; | ||||||
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@@ -175,19 +176,19 @@ let user = { | |||||
user.name = "Pete"; | ||||||
alert(user.name); // Pete | ||||||
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user.name = ""; // Name is too short... | ||||||
user.name = ""; // El nombre es demasiado corto... | ||||||
``` | ||||||
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So, the name is stored in `_name` property, and the access is done via getter and setter. | ||||||
Entonces, el nombre es almacenado en la propiedad `_name`, y el acceso se hace a traves de getter y setter. | ||||||
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Technically, external code is able to access the name directly by using `user._name`. But there is a widely known convention that properties starting with an underscore `"_"` are internal and should not be touched from outside the object. | ||||||
Técnicamente, el código externo todavía puede acceder al nombre directamente usando "usuario._nombre". Pero hay un acuerdo ampliamente conocido de que las propiedades que comienzan con un guión bajo "_" son internas y no deben ser manipuladas desde el exterior del objeto. | ||||||
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## Using for compatibility | ||||||
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One of the great uses of accessors is that they allow to take control over a "regular" data property at any moment by replacing it with a getter and a setter and tweak its behavior. | ||||||
Una de los grandes usos de los getters y setters es que permiten tomar el control de una propiedad de datos "normal" y reemplazarla con getter y setter y así refinar su coportamiento. | ||||||
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Imagine we started implementing user objects using data properties `name` and `age`: | ||||||
Imagina que empezamos a implementar objetos usuario usando las propiedades de datos "nombre" y "edad": | ||||||
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```js | ||||||
function User(name, age) { | ||||||
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@@ -200,7 +201,7 @@ let john = new User("John", 25); | |||||
alert( john.age ); // 25 | ||||||
``` | ||||||
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...But sooner or later, things may change. Instead of `age` we may decide to store `birthday`, because it's more precise and convenient: | ||||||
...Pero tarde o temprano, las cosas pueden cambiar. En lugar de "edad" podemos decidir almacenar "cumpleaños", porque es más preciso y conveniente: | ||||||
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```js | ||||||
function User(name, birthday) { | ||||||
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@@ -211,21 +212,21 @@ function User(name, birthday) { | |||||
let john = new User("John", new Date(1992, 6, 1)); | ||||||
``` | ||||||
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Now what to do with the old code that still uses `age` property? | ||||||
Ahora, ¿qué hacer con el viejo código que todavía usa la propiedad de la "edad"? | ||||||
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We can try to find all such places and fix them, but that takes time and can be hard to do if that code is used by many other people. And besides, `age` is a nice thing to have in `user`, right? | ||||||
Podemos intentar encontrar todos esos lugares y arreglarlos, pero eso lleva tiempo y puede ser difícil de hacer si ese código está escrito por otras personas. Y además, la "edad" es algo bueno para tener en "usuario", ¿verdad? En algunos lugares es justo lo que queremos. | ||||||
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Let's keep it. | ||||||
Pues mantengámoslo. | ||||||
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Adding a getter for `age` solves the problem: | ||||||
Añadiendo un getter para la "edad" resuelve el problema: | ||||||
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```js run no-beautify | ||||||
function User(name, birthday) { | ||||||
this.name = name; | ||||||
this.birthday = birthday; | ||||||
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*!* | ||||||
// age is calculated from the current date and birthday | ||||||
// La edad se calcula a partir de la fecha actual y del cumpleaños | ||||||
Object.defineProperty(this, "age", { | ||||||
get() { | ||||||
let todayYear = new Date().getFullYear(); | ||||||
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@@ -237,8 +238,8 @@ function User(name, birthday) { | |||||
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let john = new User("John", new Date(1992, 6, 1)); | ||||||
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alert( john.birthday ); // birthday is available | ||||||
alert( john.age ); // ...as well as the age | ||||||
alert( john.birthday ); // El cumpleaños está disponible | ||||||
alert( john.age ); // ...así como la edad | ||||||
``` | ||||||
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Now the old code works too and we've got a nice additional property. | ||||||
Ahora el viejo código funciona también y tenemos una buena propiedad adicional. |
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