# logical operator

[JS](https://lochiwei.gitbook.io/web/js) ⟩ [statement](https://lochiwei.gitbook.io/web/js/grammar/statement) ⟩ [expression](https://lochiwei.gitbook.io/web/js/grammar/statement/expr) ⟩ [operator](https://lochiwei.gitbook.io/web/js/grammar/op) ⟩ logical ⟩ 

{% hint style="success" %}

* <mark style="color:purple;">**`a && b`**</mark> ：<mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`a`**</mark> <mark style="color:red;">**if**</mark> <mark style="color:purple;">**`a`**</mark> is <mark style="color:red;">**falsy**</mark>, otherwise <mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`b`**</mark>.&#x20;
* <mark style="color:purple;">**`a || b`**</mark>：<mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`a`**</mark> <mark style="color:red;">**if**</mark> <mark style="color:purple;">**`a`**</mark> is <mark style="color:green;">**truthy**</mark>, otherwise <mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`b`**</mark>.&#x20;
* <mark style="color:purple;">**`a ?? b`**</mark>：<mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`a`**</mark> <mark style="color:red;">**if**</mark> <mark style="color:purple;">**`a`**</mark> is [**non-nullish**](https://lochiwei.gitbook.io/web/js/val/prim/nullish), otherwise <mark style="color:yellow;">**returns**</mark> <mark style="color:purple;">**`b`**</mark>.&#x20;
* <mark style="color:purple;">**`!a`**</mark>：<mark style="color:yellow;">**returns**</mark> <mark style="color:red;">**`false`**</mark> <mark style="color:red;">**if**</mark> <mark style="color:purple;">**`a`**</mark> is <mark style="color:green;">**truthy**</mark>, otherwise <mark style="color:yellow;">**returns**</mark> <mark style="color:green;">**`true`**</mark>.&#x20;

:star2: [table-of-operators](https://lochiwei.gitbook.io/web/js/grammar/op/table-of-operators "mention")
{% endhint %}

{% tabs %}
{% tab title="⭐️ 重點" %}
{% hint style="warning" %}

* <mark style="color:purple;">**`&&`**</mark> and <mark style="color:purple;">**`||`**</mark> can do [short-circuiting](https://lochiwei.gitbook.io/web/js/grammar/op/term/short-circuiting "mention").
* unlike <mark style="color:purple;">**`&&`**</mark> and <mark style="color:purple;">**`||`**</mark>, <mark style="color:orange;">**`!`**</mark> <mark style="color:yellow;">**always**</mark> returns <mark style="color:green;">**true**</mark> or <mark style="color:red;">**false**</mark>.
  {% endhint %}

{% hint style="success" %}
[**convert**](https://lochiwei.gitbook.io/web/js/val/type/convert) any [val](https://lochiwei.gitbook.io/web/js/val "mention") to [boolean](https://lochiwei.gitbook.io/web/js/val/prim/boolean "mention").

```javascript
!!x    // conversion: any -> true/false
```

{% endhint %}
{% endtab %}

{% tab title="👥 相關" %}

* [falsy](https://lochiwei.gitbook.io/web/js/val/type/falsy "mention") <mark style="color:yellow;">**/**</mark> [nullish](https://lochiwei.gitbook.io/web/js/val/prim/nullish "mention")
* [conditional-operator](https://lochiwei.gitbook.io/web/js/grammar/op/ternary/conditional-operator "mention")
* [optional-chaining](https://lochiwei.gitbook.io/web/js/val/obj/prop/access/optional-chaining "mention")
* [nullish-coalescing](https://lochiwei.gitbook.io/web/js/grammar/op/logical/nullish-coalescing "mention")
  {% endtab %}

{% tab title="💈範例" %}
replit：

```javascript
// Some code
```

{% endtab %}

{% tab title="📗 參考" %}

* [ ] [javascript-the-definitive-guide](https://lochiwei.gitbook.io/web/master/ref/javascript-the-definitive-guide "mention") ⟩ 4.10 Logical Expressions
  {% endtab %}

{% tab title="📘 手冊" %}
\*
{% endtab %}
{% endtabs %}


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