diff --git a/src/index.ts b/src/index.ts
index de9eeac..adea168 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -250,107 +250,80 @@ export function serialize(
   val: string,
   options?: SerializeOptions,
 ): string {
-  const enc = options?.encode || encodeURIComponent;
+  const enc = options?.encode ?? encodeURIComponent;
 
   if (!cookieNameRegExp.test(name)) {
     throw new TypeError(`argument name is invalid: ${name}`);
   }
 
   const value = enc(val);
-
   if (!cookieValueRegExp.test(value)) {
     throw new TypeError(`argument val is invalid: ${val}`);
   }
 
-  let str = name + "=" + value;
+  let str = `${name}=${value}`;
   if (!options) return str;
 
   if (options.maxAge !== undefined) {
     if (!Number.isInteger(options.maxAge)) {
       throw new TypeError(`option maxAge is invalid: ${options.maxAge}`);
     }
-
-    str += "; Max-Age=" + options.maxAge;
+    str += `; Max-Age=${options.maxAge}`;
   }
 
   if (options.domain) {
     if (!domainValueRegExp.test(options.domain)) {
       throw new TypeError(`option domain is invalid: ${options.domain}`);
     }
-
-    str += "; Domain=" + options.domain;
+    str += `; Domain=${options.domain}`;
   }
 
   if (options.path) {
     if (!pathValueRegExp.test(options.path)) {
       throw new TypeError(`option path is invalid: ${options.path}`);
     }
-
-    str += "; Path=" + options.path;
+    str += `; Path=${options.path}`;
   }
 
   if (options.expires) {
     if (
-      !isDate(options.expires) ||
+      !(options.expires instanceof Date) ||
       !Number.isFinite(options.expires.valueOf())
     ) {
       throw new TypeError(`option expires is invalid: ${options.expires}`);
     }
-
-    str += "; Expires=" + options.expires.toUTCString();
-  }
-
-  if (options.httpOnly) {
-    str += "; HttpOnly";
-  }
-
-  if (options.secure) {
-    str += "; Secure";
+    str += `; Expires=${options.expires.toUTCString()}`;
   }
 
-  if (options.partitioned) {
-    str += "; Partitioned";
-  }
+  if (options.httpOnly) str += "; HttpOnly";
+  if (options.secure) str += "; Secure";
+  if (options.partitioned) str += "; Partitioned";
 
   if (options.priority) {
+    const priorityMap = { low: "Low", medium: "Medium", high: "High" } as const;
     const priority =
-      typeof options.priority === "string"
-        ? options.priority.toLowerCase()
-        : undefined;
-    switch (priority) {
-      case "low":
-        str += "; Priority=Low";
-        break;
-      case "medium":
-        str += "; Priority=Medium";
-        break;
-      case "high":
-        str += "; Priority=High";
-        break;
-      default:
-        throw new TypeError(`option priority is invalid: ${options.priority}`);
-    }
+      priorityMap[
+        String(options.priority).toLowerCase() as keyof typeof priorityMap
+      ];
+    if (!priority)
+      throw new TypeError(`option priority is invalid: ${options.priority}`);
+    str += `; Priority=${priority}`;
   }
 
   if (options.sameSite) {
+    const sameSiteMap = {
+      strict: "Strict",
+      lax: "Lax",
+      none: "None",
+      true: "Strict",
+    } as const;
     const sameSite =
-      typeof options.sameSite === "string"
-        ? options.sameSite.toLowerCase()
-        : options.sameSite;
-    switch (sameSite) {
-      case true:
-      case "strict":
-        str += "; SameSite=Strict";
-        break;
-      case "lax":
-        str += "; SameSite=Lax";
-        break;
-      case "none":
-        str += "; SameSite=None";
-        break;
-      default:
-        throw new TypeError(`option sameSite is invalid: ${options.sameSite}`);
-    }
+      sameSiteMap[
+        String(options.sameSite).toLowerCase() as keyof typeof sameSiteMap
+      ];
+    if (!sameSite)
+      throw new TypeError(`option sameSite is invalid: ${options.sameSite}`);
+    str += `; SameSite=${sameSite}`;
   }
 
   return str;
@@ -368,10 +341,3 @@ function decode(str: string): string {
     return str;
   }
 }
-
-/**
- * Determine if value is a Date.
- */
-function isDate(val: any): val is Date {
-  return __toString.call(val) === "[object Date]";
-}