From c64895334e104ccc6cc60e872595e6338fbcdfa9 Mon Sep 17 00:00:00 2001
From: Iddar Olivares <iddar@dbug.mx>
Date: Wed, 20 Nov 2024 15:06:35 -0600
Subject: [PATCH] types: improve type inference for cx concatenation
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• Return exact string literal types instead of plain string
• Handle conditional types and unions correctly
• No runtime changes or performance impact

Before:
  cx("foo", "bar") → type: string
  cx("foo", null, "bar") → type: never

After:
  cx("foo", "bar") → type: "foo bar"
  cx("foo", null, "bar") → type: "foo bar"
---
 src/index.ts | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/index.ts b/src/index.ts
index ee6a71d..db3fa44 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,11 +1,35 @@
 export type ClassName = string | boolean | null | undefined;
 
+type FilterStrings<T extends readonly unknown[]> = T extends readonly [
+  infer F,
+  ...infer R,
+]
+  ? F extends string
+    ? [F, ...FilterStrings<R>]
+    : FilterStrings<R>
+  : [];
+
+type JoinStrings<T extends readonly string[]> = T extends readonly [
+  infer F,
+  ...infer R,
+]
+  ? F extends string
+    ? R extends readonly string[]
+      ? R["length"] extends 0
+        ? F
+        : `${F} ${JoinStrings<R>}`
+      : F
+    : never
+  : "";
+
 /**
  * Conditionally join classNames into a single string
  * @param {...String} args The expressions to evaluate
  * @returns {String} The joined classNames
  */
-function cx(...args: ClassName[]): string;
+function cx<T extends readonly ClassName[]>(
+  ...args: T
+): JoinStrings<FilterStrings<T>>;
 function cx(): string {
   let str = "",
     i = 0,