diff --git a/lib/client.js b/lib/client.js
index b753df0d906..7eb2d196d1b 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -370,10 +370,10 @@ class Client extends DispatcherBase {
     // TODO: for H2 we need to gracefully flush the remaining enqueued
     // request and close each stream.
     return new Promise((resolve) => {
-      if (!this[kSize]) {
-        resolve(null)
-      } else {
+      if (this[kSize]) {
         this[kClosedResolve] = resolve
+      } else {
+        resolve(null)
       }
     })
   }
@@ -401,10 +401,10 @@ class Client extends DispatcherBase {
         this[kHTTP2SessionState] = null
       }
 
-      if (!this[kSocket]) {
-        queueMicrotask(callback)
-      } else {
+      if (this[kSocket]) {
         util.destroy(this[kSocket].on('close', callback), err)
+      } else {
+        queueMicrotask(callback)
       }
 
       resume(this)
diff --git a/lib/core/request.js b/lib/core/request.js
index 7f8a82e6701..74e0ca16eaa 100644
--- a/lib/core/request.js
+++ b/lib/core/request.js
@@ -369,8 +369,11 @@ class Request {
 
       if (value == null || value.length === 0) continue
 
-      if (headers[key]) headers[key] += `,${value}`
-      else headers[key] = value
+      if (headers[key]) {
+        headers[key] += `,${value}`
+      } else {
+        headers[key] = value
+      }
     }
 
     return headers
@@ -433,20 +436,22 @@ function processHeader (request, key, val, skipAppend = false) {
     }
   } else if (headerName === 'expect') {
     throw new NotSupportedError('expect header not supported')
-  } else {
-    if (Array.isArray(val)) {
-      for (let i = 0; i < val.length; i++) {
-        if (skipAppend) {
-          if (request.headers[key]) request.headers[key] += `,${processHeaderValue(key, val[i], skipAppend)}`
-          else request.headers[key] = processHeaderValue(key, val[i], skipAppend)
+  } else if (Array.isArray(val)) {
+    for (let i = 0; i < val.length; i++) {
+      if (skipAppend) {
+        if (request.headers[key]) {
+          request.headers[key] += `,${processHeaderValue(key, val[i], skipAppend)}`
         } else {
-          request.headers += processHeaderValue(key, val[i])
+          request.headers[key] = processHeaderValue(key, val[i], skipAppend)
         }
+      } else {
+        request.headers += processHeaderValue(key, val[i])
       }
-    } else {
-      if (skipAppend) request.headers[key] = processHeaderValue(key, val, skipAppend)
-      else request.headers += processHeaderValue(key, val)
     }
+  } else if (skipAppend) {
+    request.headers[key] = processHeaderValue(key, val, skipAppend)
+  } else {
+    request.headers += processHeaderValue(key, val)
   }
 }
 
diff --git a/lib/core/tree.js b/lib/core/tree.js
index a61de9efbd9..03be18d235e 100644
--- a/lib/core/tree.js
+++ b/lib/core/tree.js
@@ -57,12 +57,10 @@ class TstNode {
       } else {
         this.left = new TstNode(key, value, index)
       }
+    } else if (this.right !== null) {
+      this.right.add(key, value, index)
     } else {
-      if (this.right !== null) {
-        this.right.add(key, value, index)
-      } else {
-        this.right = new TstNode(key, value, index)
-      }
+      this.right = new TstNode(key, value, index)
     }
   }
 
diff --git a/lib/core/util.js b/lib/core/util.js
index 69fe8956825..6a692465893 100644
--- a/lib/core/util.js
+++ b/lib/core/util.js
@@ -254,19 +254,19 @@ function parseHeaders (headers, obj) {
     const key = headerNameToString(headers[i])
     let val = obj[key]
 
-    if (!val) {
+    if (val) {
+      if (typeof val === 'string') {
+        val = [val]
+        obj[key] = val
+      }
+      val.push(headers[i + 1].toString('utf8'))
+    } else {
       const headersValue = headers[i + 1]
       if (typeof headersValue === 'string') {
         obj[key] = headersValue
       } else {
         obj[key] = Array.isArray(headersValue) ? headersValue.map(x => x.toString('utf8')) : headersValue.toString('utf8')
       }
-    } else {
-      if (typeof val === 'string') {
-        val = [val]
-        obj[key] = val
-      }
-      val.push(headers[i + 1].toString('utf8'))
     }
   }
 
diff --git a/lib/eventsource/eventsource-stream.js b/lib/eventsource/eventsource-stream.js
index 3117318df10..754934568d0 100644
--- a/lib/eventsource/eventsource-stream.js
+++ b/lib/eventsource/eventsource-stream.js
@@ -337,7 +337,7 @@ class EventSourceStream extends Transform {
         if (event[field] === undefined) {
           event[field] = value
         } else {
-          event[field] += '\n' + value
+          event[field] += `\n${value}`
         }
         break
       case 'retry':
diff --git a/lib/fetch/body.js b/lib/fetch/body.js
index 621c5a2d47b..45aecd04c79 100644
--- a/lib/fetch/body.js
+++ b/lib/fetch/body.js
@@ -167,7 +167,7 @@ function extractBody (object, keepalive = false) {
     // Set type to `multipart/form-data; boundary=`,
     // followed by the multipart/form-data boundary string generated
     // by the multipart/form-data encoding algorithm.
-    type = 'multipart/form-data; boundary=' + boundary
+    type = `multipart/form-data; boundary=${boundary}`
   } else if (isBlobLike(object)) {
     // Blob
 
@@ -455,7 +455,7 @@ function bodyMixinMethods (instance) {
         } catch (err) {
           // istanbul ignore next: Unclear when new URLSearchParams can fail on a string.
           // 2. If entries is failure, then throw a TypeError.
-          throw Object.assign(new TypeError(), { cause: err })
+          throw new TypeError(undefined, { cause: err })
         }
 
         // 3. Return a new FormData object whose entries are entries.
diff --git a/lib/fetch/file.js b/lib/fetch/file.js
index e4230880c92..d52feb3e3c9 100644
--- a/lib/fetch/file.js
+++ b/lib/fetch/file.js
@@ -283,12 +283,12 @@ function processBlobParts (parts, options) {
       // 2. If element is a BufferSource, get a copy of the
       //    bytes held by the buffer source, and append those
       //    bytes to bytes.
-      if (!element.buffer) { // ArrayBuffer
-        bytes.push(new Uint8Array(element))
-      } else {
+      if (element.buffer) {
         bytes.push(
           new Uint8Array(element.buffer, element.byteOffset, element.byteLength)
         )
+      } else { // ArrayBuffer
+        bytes.push(new Uint8Array(element))
       }
     } else if (isBlobLike(element)) {
       // 3. If element is a Blob, append the bytes it represents
diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index d14f4f998fd..a045bbc6dbc 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -224,9 +224,7 @@ function fetch (input, init = undefined) {
     // 3. If response is a network error, then reject p with a TypeError
     // and terminate these substeps.
     if (response.type === 'error') {
-      p.reject(
-        Object.assign(new TypeError('fetch failed'), { cause: response.error })
-      )
+      p.reject(new TypeError('fetch failed', { cause: response.error }))
       return Promise.resolve()
     }
 
diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index 3255b78dab7..37ac53015e8 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -88,14 +88,12 @@ class Response {
     try {
       parsedURL = new URL(url, getGlobalOrigin())
     } catch (err) {
-      throw Object.assign(new TypeError('Failed to parse URL from ' + url), {
-        cause: err
-      })
+      throw new TypeError(`Failed to parse URL from ${url}`, { cause: err })
     }
 
     // 3. If status is not a redirect status, then throw a RangeError.
     if (!redirectStatusSet.has(status)) {
-      throw new RangeError('Invalid status code ' + status)
+      throw new RangeError(`Invalid status code ${status}`)
     }
 
     // 4. Let responseObject be the result of creating a Response object,
@@ -472,7 +470,7 @@ function initializeResponse (response, init, body) {
     if (nullBodyStatus.includes(response.status)) {
       throw webidl.errors.exception({
         header: 'Response constructor',
-        message: 'Invalid response status code ' + response.status
+        message: `Invalid response status code ${response.status}`
       })
     }
 
diff --git a/lib/handler/RetryHandler.js b/lib/handler/RetryHandler.js
index 13eb1005083..05d55792c1f 100644
--- a/lib/handler/RetryHandler.js
+++ b/lib/handler/RetryHandler.js
@@ -151,7 +151,7 @@ class RetryHandler {
     let retryAfterHeader = headers != null && headers['retry-after']
     if (retryAfterHeader) {
       retryAfterHeader = Number(retryAfterHeader)
-      retryAfterHeader = isNaN(retryAfterHeader)
+      retryAfterHeader = Number.isNaN(retryAfterHeader)
         ? calculateRetryAfterHeader(retryAfterHeader)
         : retryAfterHeader * 1e3 // Retry-After is in seconds
     }
diff --git a/lib/websocket/receiver.js b/lib/websocket/receiver.js
index b8b63ceb9d8..c75939c29b7 100644
--- a/lib/websocket/receiver.js
+++ b/lib/websocket/receiver.js
@@ -236,9 +236,7 @@ class ByteParser extends Writable {
         }
       }
 
-      if (this.#byteOffset > 0) {
-        continue
-      } else {
+      if (this.#byteOffset === 0) {
         callback()
         break
       }
diff --git a/lib/websocket/util.js b/lib/websocket/util.js
index d15a63cde92..dc3e1ad7c3d 100644
--- a/lib/websocket/util.js
+++ b/lib/websocket/util.js
@@ -39,7 +39,7 @@ function isClosed (ws) {
  * @param {EventTarget} target
  * @param {EventInit | undefined} eventInitDict
  */
-function fireEvent (e, target, eventConstructor = Event, eventInitDict) {
+function fireEvent (e, target, eventConstructor = Event, eventInitDict = {}) {
   // 1. If eventConstructor is not given, then let eventConstructor be Event.
 
   // 2. Let event be the result of creating an event given eventConstructor,