doc: Documents existing parseCookie behavior

README.md

@@ -182,7 +182,8 @@ The default `encode` function is the global `encodeURIComponent`.
 
 The default `decode` function is the global `decodeURIComponent`, wrapped in a `try..catch`. If an error
 is thrown it will return the cookie's original value. If you provide your own encode/decode
-scheme you must ensure errors are appropriately handled.
+scheme you must ensure errors are appropriately handled. Custom decode functions can return `undefined`,
+which will skip the cookie during `parse` and try again with a future cookie of the same name.
 
 ## Example
 

src/index.ts

@@ -87,7 +87,8 @@ export interface ParseOptions {
    *
    * The default function is the global `decodeURIComponent`, wrapped in a `try..catch`. If an error
    * is thrown it will return the cookie's original value. If you provide your own encode/decode
-   * scheme you must ensure errors are appropriately handled.
+   * scheme you must ensure errors are appropriately handled. Custom decode functions can return `undefined`,
+   * which will skip the cookie during `parse` and try again with a future cookie of the same name.
    *
    * @default decode
    */

src/parse-cookie.spec.ts

@@ -110,5 +110,20 @@ describe("cookie.parseCookie", function () {
         }),
       ).toEqual({ foo: "bar" });
     });
+
+    it("should skip undefined decoded duplicates", function () {
+      let calls = 0;
+
+      expect(
+        cookie.parseCookie("foo=bar; foo=baz", {
+          decode: function (value) {
+            calls++;
+            return value === "bar" ? undefined : value;
+          },
+        }),
+      ).toEqual({ foo: "baz" });
+
+      expect(calls).toBe(2);
+    });
   });
 });