EzequielCaste · May 30, 2020 · May 18, 2020 · May 18, 2020 · May 18, 2020
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+# Nullish coalescing operator '??'
+
+[recent browser="new"]
+
+El _nullish coalescing operator_ `??` brinda una sintáxis corta para seleccionar la primera variable "definida" de una lista.
+
+El resultado de `a ?? b` es:
+- `a` si esta no es `null` o `undefined`,
+- `b`, en el caso contrario.
+
+Entonces, `x = a ?? b` es la versión corta de:
+
+```js
+x = (a !== null && a !== undefined) ? a : b;
+```
+
+Aquí un ejemplo más detallado.
+
+Pensemos que tenemos un `firstName`, `lastName` o `nickName`, todos ellos opcionales.
+
+Escojamos el que esté definido y mostrémoslo (o mostremos "Anonymous" si ninguno está definido):
+
+```js run
+let firstName = null;
+let lastName = null;
+let nickName = "Supercoder";
+
+// Muestra la primera variable que no sea null/undefined
+alert(firstName ?? lastName ?? nickName ?? "Anonymous"); // Supercoder
+```
+
+## Comparación con ||
+
+Esto es muy similiar al operador OR `||`. De hecho, podemos reemplazar `??` con `||` en el código anterior y obtener el mismo resultado.
+
+La gran diferencia es que:
+- `||` retorna el primer valor _*truthy*_.
+- `??` retorna el primer valor _*defined*_.
+
+Esto es de suma importancia cuando queremos tratar `null/undefined` diferente de `0`.
+
+Por ejemplo:
+
+```js
+height = height ?? 100;
+```
+
+Esto le asigna `100` a `height` si esta no está definida. En cambio si `height` es `0`, esta se mantiene "tal cual".
+
+Comparémoslo con `||`:
+
+```js run
+let height = 0;
+
+alert(height || 100); // 100
+alert(height ?? 100); // 0
+```
+
+En este caso, `height || 100` maneja `height` con un valor de `0` como no asignada, al igual que con `null`, `undefined` o cualquier otro valor _falsy_, dependiendo de la situación, esto puede ser incorrecto.
+
+En el caso de `height ?? 100` este retorna `100` solo si `height` es exactamente `null` o `undefined`.
+
+## Precedencia
+
+La precedencia del operador `??` es bastante baja: `7` en la [Tabla MDN](https://developer.mozilla.org/es/docs/Web/JavaScript/Referencia/Operadores/Operator_Precedence#Table).
+
+Es más baja que en la mayoría de los operadores y un poco más alta que `=` y `?`.
+
+Así que si necesitas usar `??` en una expresión compleja,  considera añadir paréntesis:
+
+```js run
+let height = null;
+let width = null;
+
+// Importante: usar paréntesis
+let area = (height ?? 100) * (width ?? 50);
+
+alert(area); // 5000
+```
+
+Caso contrario, si omitimos los paréntesis, entonces `*` tiene una mayor precedencia y se ejecutará primero. Eso sería lo mismo que:
+
+```js
+// Incorrecto
+let area = height ?? (100 * width) ?? 50;
+```
+
+Existe también una limitación a nivel del lenguaje. Por razones de seguridad, está prohibido usar `??` junto con los operadores `&&` y `||`.
+
+El siguiente código desencadena un error de sintáxis:
+
+```js run
+let x = 1 && 2 ?? 3; // Syntax error
+```
+
+La limitación es sin duda alguna debatible, pero por ciertas razones fue agregada a la especificación del lenguaje.
+
+Usa paréntesis explícitos para solucionarlo:
+
+```js run
+let x = (1 && 2) ?? 3; // Funciona
+alert(x); // 2
+```
+
+## Resumen
+
+- El _nullish coalescing operator_ `??` proveé una manera concisa de seleccionar un valor "definido" de una lista.
+
+    Es usado para asignar valores por defecto a las variables:
+
+    ```js
+    // Asignar height=100, si height es null o undefined
+    height = height ?? 100;
+    ```
+
+- El operador `??` tiene una precedencia muy baja, un poco más alta que `?` y `=`.
+- Está prohibido su uso con `||` y `&&` sin paréntesis explícitos.
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