Add mismatched placement JavaScript error (#28022)

* Add mismatched placement JavaScript error https://bugzilla.mozilla.org/show_bug.cgi?id=1707974 * Move file; style changes --------- Co-authored-by: Joshua Chen <[email protected]>
mdn · Jul 18, 2023 · 18f6260 · 18f6260
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diff --git a/...us/web/javascript/reference/errors/either_be_both_static_or_non-static/index.md b/...us/web/javascript/reference/errors/either_be_both_static_or_non-static/index.md
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+---
+title: "SyntaxError: getter and setter for private name #x should either be both static or non-static"
+slug: Web/JavaScript/Reference/Errors/Either_be_both_static_or_non-static
+page-type: javascript-error
+---
+
+{{jsSidebar("Errors")}}
+
+The JavaScript exception "mismatched placement" occurs when a private [getter](/en-US/docs/Web/JavaScript/Reference/Functions/get) and [setter](/en-US/docs/Web/JavaScript/Reference/Functions/set) are mismatched in whether or not they are {{jsxref("Classes/static", "static")}}.
+
+## Message
+
+```
+SyntaxError: Identifier '#x' has already been declared (V8-based)
+SyntaxError: getter and setter for private name #x should either be both static or non-static (Firefox)
+SyntaxError: Cannot declare a private non-static getter if there is a static private setter with used name. (Safari)
+```
+
+## Error type
+
+{{jsxref("SyntaxError")}}
+
+## What went wrong?
+
+Private [getters](/en-US/docs/Web/JavaScript/Reference/Functions/get) and [setters](/en-US/docs/Web/JavaScript/Reference/Functions/set) for the same name must either be both {{jsxref("Classes/static", "static")}}, or both non-static. This limitation does not exist for public methods.
+
+## Examples
+
+### Mismatched placement
+
+```js example-bad
+class Test {
+  static set #foo(_) {}
+  get #foo() {}
+}
+
+// SyntaxError: getter and setter for private name #foo should either be both static or non-static
+```
+
+Since `foo` is [private](/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields), the methods must be either both {{jsxref("Classes/static", "static")}}:
+
+```js example-good
+class Test {
+  static set #foo(_) {}
+  static get #foo() {}
+}
+```
+
+or non-static:
+
+```js example-good
+class Test {
+  set #foo(_) {}
+  get #foo() {}
+}
+```
+
+## See also
+
+- {{jsxref("Functions/get", "get")}}
+- {{jsxref("Functions/set", "set")}}
+- {{jsxref("Classes/static", "static")}}
+- [Private class features](/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields)