# assignment (=)

[JS](https://lochiwei.gitbook.io/web/js) ⟩ [operator](https://lochiwei.gitbook.io/web/js/grammar/op) ⟩ [assignment](https://lochiwei.gitbook.io/web/js/grammar/op/assign) ⟩ <mark style="color:blue;">`=`</mark>

{% hint style="success" %}
([..](https://lochiwei.gitbook.io/web/js/grammar/op "mention")) <mark style="color:yellow;">**assigns**</mark> [val](https://lochiwei.gitbook.io/web/js/val "mention") to [variable](https://lochiwei.gitbook.io/web/js/variable "mention") <mark style="color:yellow;">**/**</mark> [prop](https://lochiwei.gitbook.io/web/js/val/obj/prop "mention") <mark style="color:yellow;">**/**</mark> [element](https://lochiwei.gitbook.io/web/js/val/builtin/arr/element "mention"), and <mark style="color:yellow;">**returns**</mark> the <mark style="color:orange;">**assigned**</mark>**&#x20;**<mark style="color:yellow;">**value**</mark>.
{% endhint %}

{% tabs %}
{% tab title="⭐️ 重點" %}
{% hint style="warning" %}
[var](https://lochiwei.gitbook.io/web/js/variable/declare/var "mention") [<mark style="color:yellow;">**redeclaration**</mark>](https://lochiwei.gitbook.io/web/js/variable/declare)&#x20;

* is <mark style="color:red;">**ignored**</mark> and <mark style="color:red;">**won't**</mark> trigger any <mark style="color:yellow;">**error**</mark>, <mark style="color:red;">**even in**</mark> [strict-mode](https://lochiwei.gitbook.io/web/js/concept/env/js-engine/mode/strict-mode "mention"):exclamation:
* ⭐️ the <mark style="color:purple;">**assignment (=)**</mark> part is <mark style="color:red;">**applied**</mark>:exclamation:

👉 [redeclare](https://lochiwei.gitbook.io/web/js/variable/declare/var/redeclare "mention")
{% endhint %}

{% hint style="warning" %}
⭐️ <mark style="color:purple;">**assignment (=)**</mark> to an [undeclared](https://lochiwei.gitbook.io/web/js/grammar/token/id/undeclared "mention") will result in&#x20;

* :exclamation:[accidental-global-variable-in-sloppy-mode](https://lochiwei.gitbook.io/web/js/concept/env/js-engine/mode/sloppy-mode/accidental-global-variable-in-sloppy-mode "mention") in [sloppy-mode](https://lochiwei.gitbook.io/web/js/concept/env/js-engine/mode/sloppy-mode "mention").
* :shield:[ref](https://lochiwei.gitbook.io/web/js/err/ref "mention") ⟩ [not-defined](https://lochiwei.gitbook.io/web/js/err/ref/not-defined "mention") in [strict-mode](https://lochiwei.gitbook.io/web/js/concept/env/js-engine/mode/strict-mode "mention")
  {% endhint %}

{% hint style="info" %}
the <mark style="color:purple;">**assignment**</mark> of a [**property**](https://lochiwei.gitbook.io/web/js/val/obj/prop) <mark style="color:red;">**always**</mark> <mark style="color:yellow;">**creates**</mark>**/**<mark style="color:yellow;">**sets a property**</mark> in the <mark style="color:orange;">**original**</mark>**&#x20;**<mark style="color:yellow;">**object**</mark>, it <mark style="color:red;">**never**</mark> <mark style="color:yellow;">**modifies objects**</mark> in the [**prototype chain**](https://lochiwei.gitbook.io/web/js/val/obj/proto/chain).
{% endhint %}
{% endtab %}

{% tab title="🔴 主題" %}

* [destruct](https://lochiwei.gitbook.io/web/js/grammar/op/assign/destruct "mention")
* [dot-notation-.](https://lochiwei.gitbook.io/web/js/val/obj/prop/access/dot-notation-. "mention") / [bracket-notation](https://lochiwei.gitbook.io/web/js/val/obj/prop/access/bracket-notation "mention") - assignment of a property
  {% endtab %}

{% tab title="👥 相關" %}

* [strict-equality](https://lochiwei.gitbook.io/web/js/grammar/op/relational/equal/strict-equality "mention")
* [sloppy-equality](https://lochiwei.gitbook.io/web/js/grammar/op/relational/equal/sloppy-equality "mention")
* [lvalue](https://lochiwei.gitbook.io/web/js/grammar/op/term/lvalue "mention") is a term for  [variable](https://lochiwei.gitbook.io/web/js/variable "mention") <mark style="color:yellow;">**/**</mark> [prop](https://lochiwei.gitbook.io/web/js/val/obj/prop "mention") <mark style="color:yellow;">**/**</mark> [element](https://lochiwei.gitbook.io/web/js/val/builtin/arr/element "mention").
  {% endtab %}

{% tab title="📗 參考" %}

* [javascript-the-definitive-guide](https://lochiwei.gitbook.io/web/master/ref/javascript-the-definitive-guide "mention") ⟩ 4.11 Assignment Expressions
  {% endtab %}

{% tab title="📘 手冊" %}

* [Expressions and operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators) ⟩ [assignment operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators#assignment_operators) ⟩ [Assignment (=)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Assignment)
  {% endtab %}
  {% endtabs %}


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