feat: serialize URL objects over RPC

.changeset/serialize-url.md

@@ -0,0 +1,5 @@
+---
+"capnweb": minor
+---
+
+Support serializing `URL` objects over RPC.

README.md

@@ -203,6 +203,7 @@ The following types can be passed over RPC (in arguments or return values), and
 * `Error` and its well-known subclasses
 * `Blob`
 * `ReadableStream` and `WritableStream`, with automatic flow control.
+* `URL`
 * `Headers`, `Request`, and `Response` from the Fetch API.
 
 The following types are not supported as of this writing, but may be added in the future:

__tests__/index.test.ts

@@ -39,6 +39,8 @@ let SERIALIZE_TEST_CASES: Record<string, unknown> = {
   '["-inf"]': -Infinity,
   '["nan"]': NaN,
 
+  '["url","https://example.com/path?q=1"]': new URL("https://example.com/path?q=1"),
+
   '["headers",[]]': new Headers(),
   '["headers",[["content-type","text/plain"],["x-custom","hello"]]]':
       new Headers({"Content-Type": "text/plain", "X-Custom": "hello"}),
@@ -80,7 +82,7 @@ describe("simple serialization", () => {
   it("can deserialize", () => {
     for (let key in SERIALIZE_TEST_CASES) {
       let value = deserialize(key);
-      if (value instanceof Uint8Array ||
+      if (value instanceof Uint8Array || value instanceof URL ||
           value instanceof Headers || value instanceof Request || value instanceof Response) {
         // toStrictEqual() won't work for these (e.g. in Node.js, Uint8Array may deserialize as
         // Buffer), so test by serializing again and making sure they round-trip.

src/core.ts

@@ -39,7 +39,7 @@ export type PropertyPath = (string | number)[];
 
 type TypeForRpc = "unsupported" | "primitive" | "object" | "function" | "array" | "date" |
     "bigint" | "bytes" | "blob" | "stub" | "rpc-promise" | "rpc-target" | "rpc-thenable" |
-    "error" | "undefined" | "writable" | "readable" | "headers" | "request" | "response";
+    "error" | "undefined" | "writable" | "readable" | "url" | "headers" | "request" | "response";
 
 const AsyncFunction = (async function () {}).constructor;
 
@@ -115,6 +115,9 @@ export function typeForRpc(value: unknown): TypeForRpc {
     case ReadableStream.prototype:
       return "readable";
 
+    case URL.prototype:
+      return "url";
+
     case Headers.prototype:
       return "headers";
 
@@ -963,6 +966,7 @@ export class RpcPayload {
       case "date":
       case "bytes":
       case "blob":
+      case "url":
       case "error":
       case "undefined":
         // immutable, no need to copy
@@ -1344,6 +1348,7 @@ export class RpcPayload {
       case "bytes":
       case "blob":
       case "date":
+      case "url":
       case "error":
       case "undefined":
         return;
@@ -1489,6 +1494,7 @@ export class RpcPayload {
       case "rpc-target":
       case "writable":
       case "readable":
+      case "url":
       case "headers":
       case "request":
       case "response":
@@ -1642,6 +1648,7 @@ function followPath(value: unknown, parent: object | undefined,
       case "blob":
       case "date":
       case "error":
+      case "url":
       case "headers":
       case "request":
       case "response":

src/serialize.ts

@@ -377,6 +377,13 @@ export class Devaluator {
         return alternateTypeName === undefined ? ["bytes", b64] : ["bytes", b64, alternateTypeName];
       }
 
+      case "url":
+        // At structuredClonable level, keep URL as native value.
+        if (this.encodingLevel === "structuredClonable") {
+          return value;
+        }
+        return ["url", (<URL>value).href];
+
       case "headers":
         // The `Headers` TS type apparently doesn't declare itself as being
         // Iterable<[string, string]>, but it is.
@@ -950,6 +957,12 @@ export class Evaluator {
         case "nan":
           return NaN;
 
+        case "url":
+          if (value.length === 2 && typeof value[1] === "string") {
+            return new URL(value[1]);
+          }
+          break;
+
         case "headers":
           // We only need to validate that the parameter is an array, so as not to invoke an
           // unexpected variant of the Headers constructor. So long as it is an array then we can