Syntax Lookup: Add optional args

Author: fhammerschmidt

misc_docs/syntax/language_optional_labeled_argument.mdx

@@ -0,0 +1,48 @@
+---
+id: "optional-labeled-argument"
+keywords: ["optional", "labeled", "argument"]
+name: "~arg=?"
+summary: "This is an `optional labeled argument`."
+category: "languageconstructs"
+---
+
+Labeled arguments, i.e. arguments that are prefixed with `~`, can be suffixed with `=?` to denote that they are optional. Thus, they can be
+omitted when calling the function.
+
+### Example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let print = (text, ~logLevel=?) => {
+  switch logLevel {
+  | Some(#error) => Console.error(text)
+  | _ => Console.log(text)
+  }
+}
+
+print("An info")
+print("An error", ~logLevel=#error)
+```
+
+```js
+function print(text, logLevel) {
+  if (logLevel === "error") {
+    console.error(text);
+  } else {
+    console.log(text);
+  }
+}
+
+print("An info", undefined);
+
+print("An error", "error");
+```
+
+</CodeTab>
+
+### References
+
+* [Labeled Arguments](/docs/manual/latest/function#labeled-arguments)
+* [Optional Labeled Arguments](/docs/manual/latest/function#optional-labeled-arguments)
+* [Function Syntax Cheatsheet](/docs/manual/latest/function#tips--tricks)