Set of() to be nullable in ReceiptCode (#21)

Author: cable8mm

src/ReceiptCode.php

@@ -82,12 +82,12 @@ public function nextCode(): string
     /**
      * Create a new instance of ReceiptCode.
      *
-     * @param  string  $code  ReceiptCode
+     * @param  string|null  $code  ReceiptCode
      * @return static Provides a new instance of ReceiptCode
      */
-    public static function of(string $code): self
+    public static function of(?string $code = null): self
     {
-        return new self($code);
+        return is_null($code) ? self::make() : new self($code);
     }
 
     /**

tests/ReceiptCodeTest.php

@@ -29,5 +29,7 @@ public function test_next_code_method()
     public function test_no_code()
     {
         $this->assertEquals('PO-'.date('Ymd').'-0001', ReceiptCode::make()->nextCode());
+
+        $this->assertEquals('PO-'.date('Ymd').'-0001', ReceiptCode::of()->nextCode());
     }
 }