diff --git a/1-js/02-first-steps/12-nullish-coalescing-operator/article.md b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md new file mode 100644 index 000000000..2621cf970 --- /dev/null +++ b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md @@ -0,0 +1,117 @@ +# 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. \ No newline at end of file diff --git a/1-js/02-first-steps/12-while-for/1-loop-last-value/solution.md 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