> For the complete documentation index, see [llms.txt](https://lochiwei.gitbook.io/web/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lochiwei.gitbook.io/web/js/concept/env/js-engine/mode/sloppy-mode.md).

# sloppy mode

[JS](/web/js.md) ⟩ [concepts](/web/browser/concepts.md) ⟩ [environment](/web/js/concept/env.md) ⟩ [JS engine](/web/js/concept/env/js-engine.md) ⟩ [mode](/web/js/concept/env/js-engine/mode.md) ⟩ sloppy

{% hint style="success" %}
:u6307: synonyms： "<mark style="color:purple;">**non-strict mode**</mark>", "<mark style="color:purple;">**sloppy mode**</mark>"

:point\_right: compare： [strict mode](/web/js/concept/env/js-engine/mode/strict-mode.md)
{% endhint %}

{% tabs %}
{% tab title="⭐️ 重點" %}
{% hint style="warning" %} <mark style="color:red;">**in**</mark> <mark style="color:purple;">**sloppy mode**</mark>：

* although [scope](/web/js/scope.md)s are <mark style="color:yellow;">**determined**</mark> at [compile-time](/web/js/compile/compile-time.md), you can <mark style="color:red;">**modify**</mark> the [scope](/web/js/scope.md) at [runtime](/web/js/compile/runtime.md):exclamation: ( 👉 [modify current scope at runtime❗️](/web/js/concept/env/js-engine/mode/sloppy-mode/modify-current-scope-at-runtime.md):exclamation:)
* [function in block (FiB)](/web/js/val/func/declare/function-in-block-fib.md) is [<mark style="color:yellow;">**hoisted**</mark>](/web/js/scope/hoist/function.md) / <mark style="color:yellow;">**initialized**</mark> to [undefined](/web/js/val/prim/undefined.md) (<mark style="color:orange;">**outside the**</mark> [block](/web/js/grammar/statement/other/block.md)):exclamation:
* [assignment (=)](/web/js/grammar/op/assign/assignment.md) to [undeclared identifier](/web/js/grammar/token/id/undeclared.md) will result in [accidental global variable in sloppy mode❗️](/web/js/concept/env/js-engine/mode/sloppy-mode/accidental-global-variable-in-sloppy-mode.md)
  {% endhint %}

{% hint style="warning" %}
JS is <mark style="color:green;">**lenient**</mark> with [var](/web/js/variable/declare/var.md), <mark style="color:red;">**strict**</mark> with [let](/web/js/variable/declare/let.md).

* [var redeclaration applied even in strict mode❗️](/web/js/variable/declare/var/redeclare.md), even in [strict mode](/web/js/concept/env/js-engine/mode/strict-mode.md):exclamation:
* [let redeclaration not allowed even in sloppy mode❗️](/web/js/variable/declare/let/no-redeclare.md), even in [sloppy mode](/web/js/concept/env/js-engine/mode/sloppy-mode.md):exclamation:
  {% endhint %}

{% hint style="warning" %}
with a [function in block (FiB)](/web/js/val/func/declare/function-in-block-fib.md)

* [sloppy mode](/web/js/concept/env/js-engine/mode/sloppy-mode.md): ✅ [<mark style="color:yellow;">**hoisted**</mark>](/web/js/scope/hoist/function.md) / <mark style="color:yellow;">**initialized**</mark> to [undefined](/web/js/val/prim/undefined.md) (in <mark style="color:orange;">**outer**</mark> [scope](/web/js/scope.md))
* [strict mode](/web/js/concept/env/js-engine/mode/strict-mode.md): ⛔ [<mark style="color:red;">**ReferenceError**</mark>](/web/js/err/ref/not-defined.md) (<mark style="color:yellow;">**invisible**</mark> in <mark style="color:orange;">**outer**</mark> [scope](/web/js/scope.md)):exclamation:
  {% endhint %}
  {% endtab %}

{% tab title="🔴 主題" %}

* [accidental global variable in sloppy mode❗️](/web/js/concept/env/js-engine/mode/sloppy-mode/accidental-global-variable-in-sloppy-mode.md):exclamation:
* [modify current scope at runtime❗️](/web/js/concept/env/js-engine/mode/sloppy-mode/modify-current-scope-at-runtime.md):exclamation:
  {% endtab %}

{% tab title="🖥️ 影片" %}
{% embed url="<https://youtu.be/G9QTBS2x8U4>" %}
{% endtab %}

{% tab title="📘 手冊" %}

* [ ] [Strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode)
* [ ] [Sloppy mode](https://developer.mozilla.org/en-US/docs/Glossary/Sloppy_mode)
  {% endtab %}
  {% endtabs %}