# function declaration instantiation

[JS](/web/js.md) ⟩ [value](/web/js/val.md) ⟩ [function](/web/js/val/func.md) ⟩ [declaration](/web/js/val/func/declare.md) ⟩ instantiation

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🚧 
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{% tab title="⭐️ 重點" %}
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:star: [function declaration](/web/js/val/func/declare.md) <mark style="color:purple;">**instantiation**</mark>

When an [execution context](/web/js/concept/execution-context.md) is established for <mark style="color:yellow;">**evaluating**</mark> a [function](/web/js/val/func.md)：

* a <mark style="color:red;">**new**</mark> [Function Environment Record](https://tc39.es/ecma262/#sec-function-environment-records) is <mark style="color:yellow;">**created**</mark>.&#x20;
* <mark style="color:yellow;">**bindings**</mark> for each [parameter](/web/js/val/func/param.md) are <mark style="color:yellow;">**instantiated**</mark> in that [Environment Record](https://tc39.es/ecma262/#sec-environment-records).&#x20;
* <mark style="color:green;">**body declarations**</mark> are also instantiated：
  * :star: if the <mark style="color:yellow;">**parameters**</mark> <mark style="color:red;">**do not**</mark>**&#x20;**<mark style="color:yellow;">**include**</mark> any <mark style="color:yellow;">**default value initializers**</mark>： \
    the <mark style="color:green;">**body declarations**</mark> are instantiated in the <mark style="color:red;">**same**</mark> [Environment Record](https://tc39.es/ecma262/#sec-environment-records) as the <mark style="color:orange;">**parameters**</mark>:exclamation:(:point\_right: [parameter scope](/web/js/scope/parameter-scope.md))
  * :star: if <mark style="color:yellow;">**default value initializers**</mark> <mark style="color:red;">**exist**</mark>：

    a <mark style="color:red;">**second**</mark> [Environment Record](https://tc39.es/ecma262/#sec-environment-records) is created for the <mark style="color:green;">**body declarations**</mark>:exclamation:
* [parameter](/web/js/val/func/param.md)(s) / [function](/web/js/val/func.md)(s) are initialized as <mark style="color:yellow;">**part**</mark> of [FunctionDeclarationInstantiation](https://tc39.es/ecma262/#sec-functiondeclarationinstantiation). All <mark style="color:yellow;">**other bindings**</mark> are initialized during <mark style="color:yellow;">**evaluation**</mark> of the <mark style="color:yellow;">**function body**</mark>.

📘 [JS spec](/web/master/ref/js-spec.md) ⟩ [FunctionDeclarationInstantiation](https://tc39.es/ecma262/#sec-functiondeclarationinstantiation)
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{% tab title="👥 相關" %}

* [redeclaration](/web/js/grammar/declare/redeclare.md)
* [variable redeclaration](/web/js/variable/redeclare.md)
* [simple vs. non-simple parameter](/web/js/val/func/param/simple-vs-nonsimple.md) - <mark style="color:yellow;">**determines**</mark> whether <mark style="color:blue;">**parameters**</mark> and <mark style="color:green;">**body declarations**</mark> are <mark style="color:yellow;">**stored**</mark> in the <mark style="color:red;">**same**</mark> <mark style="color:yellow;">**environment record**</mark>.
* [parameter scope](/web/js/scope/parameter-scope.md) - the scope where all the parameters live.
* [execution context](/web/js/concept/execution-context.md) ⟩ [environment record](/web/js/concept/execution-context/lexical-environment/environment-record.md)
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{% tab title="📘 手冊" %}

* [JS spec](/web/master/ref/js-spec.md) ⟩ [FunctionDeclarationInstantiation](https://tc39.es/ecma262/#sec-functiondeclarationinstantiation)
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{% 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) ⟩ Appendix A
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{% tab title="❓" %}
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Q：

* does "evaluating a function" mean "calling a function"❓
* or, it means "evaluating the function itself" (to be a "function" object)❓
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[execution context](/web/js/concept/execution-context.md) 是 [runtime](/web/js/compile/runtime.md) 的才有的動作，但 evaluating the function to be a "function" object 是 compile-time 的問題，所以我猜答案應該是第一個吧❓
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{% tab title="🚧" %}

* [ ] binding
* [ ] default value initializer
* [ ] function environment record
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