# typeof let/const/class in TDZ gets an error❗️

* [JS](/web/js.md) ⟩ [value](/web/js/val.md) ⟩ [type](/web/js/val/type.md) ⟩ [name](/web/js/val/type/name.md) ⟩ [typeof](/web/js/val/type/name/typeof.md) ⟩ no typeof let/const/class in TDZ
* [JS](/web/js.md) ⟩ [operator](/web/js/grammar/op.md) ⟩ [unary](/web/js/grammar/op/unary.md) ⟩ [typeof](/web/js/val/type/name/typeof.md) ⟩ no typeof let/const/class in TDZ

{% hint style="danger" %} <mark style="color:yellow;">**using**</mark> [typeof](/web/js/val/type/name/typeof.md) <mark style="color:yellow;">**on**</mark> [let](/web/js/variable/declare/let.md)/ [const](/web/js/variable/declare/const.md)/ [class](/web/js/val/class.md) in its [temporal dead zone](/web/js/variable/access/tdz.md) will <mark style="color:yellow;">**throw**</mark> an [<mark style="color:red;">**error**</mark>](/web/js/err/ref/cannot-access-before-init.md).
{% endhint %}

{% hint style="danger" %}
:no\_entry: [<mark style="color:red;">**ReferenceError**</mark>](/web/js/err/ref.md)：[<mark style="color:yellow;">**cannot access '...' before initialization**</mark>](/web/js/err/ref/cannot-access-before-init.md):exclamation:
{% endhint %}

{% tabs %}
{% tab title="🧨 雷區" %}
replit：[typeof in TDZ](https://replit.com/@pegasusroe/typeof-in-TDZ#index.js)

```javascript
'use strict';    // ⭐ toggle sloppy/strict mode

const { log } = console;

// ┌───────────────────────┐
// │  temporal dead zone   │
// └───────────────────────┘

// ❗ lexical declarations (let/const/class) are still "uninitialized"
typeof aLet;       // ⛔ ReferenceError
typeof aConst;     // ⛔ ReferenceError
typeof aClass;     // ⛔ ReferenceError
// ⛔ ReferenceError: Cannot access '...' before initialization

// ┌───────────────────────┐
// │ undeclared identifier │
// └───────────────────────┘

// ⭐ can't reference an "undeclared identifier" direcctly.
undeclared;        // ⛔ ReferenceError
// ⛔ ReferenceError: 'undeclared' is not defined

// ⭐ but `typeof` can be used with "undeclared identifier"
typeof undeclared;                        // ❗ 'undefined'

// ❗ "var" has no "temporal dead zone", but it's `undefined` initially.
log(aVar === undefined, typeof aVar);     // ❗ true, 'undefined'

// ┌────────────────────────────────────────┐
// │ lexical declarations (let/const/class) │
// └────────────────────────────────────────┘

let aLet;                                 // ⭐ initialized to `undefined`
const aConst = "hello";
class aClass {}

// ┌─────┐
// │ var │
// └─────┘

// ❗ "var" is a "statement", not "declaration".
var aVar;
```

{% endtab %}

{% tab title="⭐️ 重點" %}
{% hint style="danger" %}

```javascript
typeof null === "object"    // true
```

{% endhint %}
{% endtab %}

{% tab title="👥 相關" %}

* :floppy\_disk:[typeName()](/web/js/val/type/name/typename.md) - <mark style="color:yellow;">**refined**</mark> version of <mark style="color:purple;">**typeof**</mark>.
* :u6307:[lexical declaration](/web/js/grammar/declare/lexical-declaration.md) - let / const / class.
* :no\_entry:[cannot access 'xxx' before initialization❗️](/web/js/err/ref/cannot-access-before-init.md)
  {% endtab %}

{% tab title="📘 手冊" %}

* [typeof](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof)
  {% endtab %}
  {% endtabs %}


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