# uninitialized variable

[JS](/web/js.md) ⟩ [variable](/web/js/variable.md) ⟩ uninitialized variable

{% hint style="success" %} <mark style="color:yellow;">**when**</mark> a [let](/web/js/variable/declare/let.md) / [const](/web/js/variable/declare/const.md) / [class](/web/js/val/class.md) is still in its [temporal dead zone](/web/js/variable/access/tdz.md), it is said to be <mark style="color:purple;">**uninitialized**</mark>.
{% endhint %}

{% tabs %}
{% tab title="⭐️ 重點" %}
{% hint style="info" %} <mark style="color:yellow;">**TDZ**</mark> vs. <mark style="color:purple;">**uninitialized variable**</mark>

* [TDZ](/web/js/variable/access/tdz.md) refers to the <mark style="color:yellow;">**time window (time period)**</mark> where a [const](/web/js/variable/declare/const.md)/[let](/web/js/variable/declare/let.md) variable exists but is still <mark style="color:purple;">**uninitialized**</mark> (and <mark style="color:red;">**cannot**</mark>**&#x20;**<mark style="color:yellow;">**be accessed**</mark> in any way).
* <mark style="color:purple;">**uninitialized variable**</mark> refers to the [const](/web/js/variable/declare/const.md)/[let](/web/js/variable/declare/let.md) variable itself.

they are two sides of the same coin.
{% endhint %}
{% endtab %}

{% tab title="👥 相關" %}

* <mark style="color:purple;">**uninitialized variable**</mark> has <mark style="color:red;">**no**</mark> [initial value](/web/js/variable/declare/initial-value.md), <mark style="color:red;">**not**</mark> [undefined](/web/js/val/prim/undefined.md) <mark style="color:yellow;">**even**</mark>:exclamation:
  {% endtab %}

{% tab title="📗 參考" %}

* [ ] [YDKJS: Scope & Closures (v.2)](/web/master/ref/book/you-dont-know-js-series-v.2/ydkjs-scope-and-closures-v.2.md) ⟩&#x20;
  * [ ] [Ch. 1: What's the Scope?](https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/scope-closures/ch1.md#chapter-1-whats-the-scope) ⟩ [Hoisting](https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/scope-closures/ch1.md#hoisting)
  * [ ] Ch. 5 > [Uninitialized Variables (aka, TDZ)](https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/scope-closures/ch5.md#uninitialized-variables-aka-tdz)
    {% endtab %}

{% tab title="📘 手冊" %}

* [ ] [temporal dead zone](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#temporal_dead_zone_tdz)
  {% endtab %}
  {% endtabs %}


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