diff --git a/lib/core/request.js b/lib/core/request.js
index 376b360dfdb..c9bf388c316 100644
--- a/lib/core/request.js
+++ b/lib/core/request.js
@@ -15,7 +15,7 @@ const {
   isIterable,
   isBlobLike,
   serializePathWithQuery,
-  validateHandler,
+  assertRequestHandler,
   getServerName,
   normalizedMethodRecords
 } = require('./util')
@@ -183,7 +183,7 @@ class Request {
       throw new InvalidArgumentError('headers must be an object or an array')
     }
 
-    validateHandler(handler, method, upgrade)
+    assertRequestHandler(handler, method, upgrade)
 
     this.servername = servername || getServerName(this.host) || null
 
diff --git a/lib/core/util.js b/lib/core/util.js
index cdc73195714..c5676ae6b64 100644
--- a/lib/core/util.js
+++ b/lib/core/util.js
@@ -345,7 +345,7 @@ function isDestroyed (body) {
 /**
  * @param {import ('stream').Stream} stream
  * @param {Error} [err]
- * @returns
+ * @returns {void}
  */
 function destroy (stream, err) {
   if (stream == null || !isStream(stream) || isDestroyed(stream)) {
@@ -478,12 +478,24 @@ function parseRawHeaders (headers) {
   return ret
 }
 
+/**
+ * @param {*} buffer
+ * @returns {buffer is Buffer}
+ */
 function isBuffer (buffer) {
   // See, https://github.com/mcollina/undici/pull/319
   return buffer instanceof Uint8Array || Buffer.isBuffer(buffer)
 }
 
-function validateHandler (handler, method, upgrade) {
+/**
+ * Asserts that the handler object is a request handler.
+ *
+ * @param {object} handler
+ * @param {string} method
+ * @param {string} [upgrade]
+ * @returns {asserts handler is import('../api/api-request').RequestHandler}
+ */
+function assertRequestHandler (handler, method, upgrade) {
   if (!handler || typeof handler !== 'object') {
     throw new InvalidArgumentError('handler must be an object')
   }
@@ -560,6 +572,7 @@ function getSocketInfo (socket) {
 }
 
 /**
+ * @param {Iterable} iterable
  * @returns {ReadableStream}
  */
 function ReadableStreamFrom (iterable) {
@@ -623,23 +636,51 @@ function addAbortListener (signal, listener) {
   return () => signal.removeListener('abort', listener)
 }
 
-const hasToWellFormed = typeof String.prototype.toWellFormed === 'function'
-const hasIsWellFormed = typeof String.prototype.isWellFormed === 'function'
-
 /**
- * @param {string} val
+ * @function
+ * @param {string} value
+ * @returns {string}
  */
-function toUSVString (val) {
-  return hasToWellFormed ? `${val}`.toWellFormed() : nodeUtil.toUSVString(val)
-}
+const toUSVString = (() => {
+  if (typeof String.prototype.toWellFormed === 'function') {
+    /**
+     * @function
+     * @param {string} value
+     * @returns {string}
+     */
+    return (value) => `${value}`.toWellFormed()
+  } else {
+    /**
+     * @function
+     * @param {string} value
+     * @returns {string}
+     */
+    return nodeUtil.toUSVString
+  }
+})()
 
 /**
- * @param {string} val
+ * @param {*} value
+ * @returns {boolean}
  */
 // TODO: move this to webidl
-function isUSVString (val) {
-  return hasIsWellFormed ? `${val}`.isWellFormed() : toUSVString(val) === `${val}`
-}
+const isUSVString = (() => {
+  if (typeof String.prototype.isWellFormed === 'function') {
+    /**
+     * @function
+     * @param {*} value
+     * @returns {boolean}
+     */
+    return (value) => `${value}`.isWellFormed()
+  } else {
+    /**
+     * @function
+     * @param {*} value
+     * @returns {boolean}
+     */
+    return (value) => toUSVString(value) === `${value}`
+  }
+})()
 
 /**
  * @see https://tools.ietf.org/html/rfc7230#section-3.2.6
@@ -710,11 +751,18 @@ function isValidHeaderValue (characters) {
 
 const rangeHeaderRegex = /^bytes (\d+)-(\d+)\/(\d+)?$/
 
+/**
+ * @typedef {object} RangeHeader
+ * @property {number} start
+ * @property {number | null} end
+ * @property {number | null} size
+ */
+
 /**
  * Parse accordingly to RFC 9110
  * @see https://www.rfc-editor.org/rfc/rfc9110#field.content-range
  * @param {string} [range]
- * @returns
+ * @returns {RangeHeader|null}
  */
 function parseRangeHeader (range) {
   if (range == null || range === '') return { start: 0, end: null, size: null }
@@ -730,9 +778,11 @@ function parseRangeHeader (range) {
 }
 
 /**
- * @param {Record<string|symbol, any>} obj
+ * @template {import("events").EventEmitter} T
+ * @param {T} obj
  * @param {string} name
- * @param {Function} listener
+ * @param {(...args: any[]) => void} listener
+ * @returns {T}
  */
 function addListener (obj, name, listener) {
   const listeners = (obj[kListeners] ??= [])
@@ -742,7 +792,9 @@ function addListener (obj, name, listener) {
 }
 
 /**
- * @param {Record<string|symbol, any>} obj
+ * @template {import("events").EventEmitter} T
+ * @param {T} obj
+ * @returns {T}
  */
 function removeAllListeners (obj) {
   if (obj[kListeners] != null) {
@@ -751,6 +803,7 @@ function removeAllListeners (obj) {
     }
     obj[kListeners] = null
   }
+  return obj
 }
 
 /**
@@ -821,7 +874,7 @@ module.exports = {
   deepClone,
   ReadableStreamFrom,
   isBuffer,
-  validateHandler,
+  assertRequestHandler,
   getSocketInfo,
   isFormDataLike,
   serializePathWithQuery,
diff --git a/lib/handler/redirect-handler.js b/lib/handler/redirect-handler.js
index 3db7a476d45..7db5eaf385e 100644
--- a/lib/handler/redirect-handler.js
+++ b/lib/handler/redirect-handler.js
@@ -38,7 +38,7 @@ class RedirectHandler {
       throw new InvalidArgumentError('maxRedirections must be a positive number')
     }
 
-    util.validateHandler(handler, opts.method, opts.upgrade)
+    util.assertRequestHandler(handler, opts.method, opts.upgrade)
 
     this.dispatch = dispatch
     this.location = null