diff --git a/src/ui/js/diff2html-ui-base.ts b/src/ui/js/diff2html-ui-base.ts
index 031d008..20b10e8 100644
--- a/src/ui/js/diff2html-ui-base.ts
+++ b/src/ui/js/diff2html-ui-base.ts
@@ -52,10 +52,13 @@ export class Diff2HtmlUI {
 
   synchronisedScroll(): void {
     this.targetElement.querySelectorAll('.d2h-file-wrapper').forEach(wrapper => {
-      const [left, right] = [].slice.call(wrapper.querySelectorAll('.d2h-file-side-diff')) as HTMLElement[];
+      const [left, right] = Array<Element>().slice.call(wrapper.querySelectorAll('.d2h-file-side-diff'));
+
       if (left === undefined || right === undefined) return;
+
       const onScroll = (event: Event): void => {
         if (event === null || event.target === null) return;
+
         if (event.target === left) {
           right.scrollTop = left.scrollTop;
           right.scrollLeft = left.scrollLeft;
@@ -70,29 +73,28 @@ export class Diff2HtmlUI {
   }
 
   fileListToggle(startVisible: boolean): void {
-    const hashTag = this.getHashTag();
-
-    const showBtn = this.targetElement.querySelector('.d2h-show') as HTMLElement;
-    const hideBtn = this.targetElement.querySelector('.d2h-hide') as HTMLElement;
-    const fileList = this.targetElement.querySelector('.d2h-file-list') as HTMLElement;
+    const showBtn: HTMLElement | null = this.targetElement.querySelector('d2h-show');
+    const hideBtn: HTMLElement | null = this.targetElement.querySelector('.d2h-hide');
+    const fileList: HTMLElement | null = this.targetElement.querySelector('.d2h-file-list');
 
     if (showBtn === null || hideBtn === null || fileList === null) return;
 
-    function show(): void {
+    const show: () => void = () => {
       showBtn.style.display = 'none';
       hideBtn.style.display = 'inline';
       fileList.style.display = 'block';
-    }
+    };
 
-    function hide(): void {
+    const hide: () => void = () => {
       showBtn.style.display = 'inline';
       hideBtn.style.display = 'none';
       fileList.style.display = 'none';
-    }
+    };
 
     showBtn.addEventListener('click', () => show());
     hideBtn.addEventListener('click', () => hide());
 
+    const hashTag = this.getHashTag();
     if (hashTag === 'files-summary-show') show();
     else if (hashTag === 'files-summary-hide') hide();
     else if (startVisible) show();
@@ -117,11 +119,11 @@ export class Diff2HtmlUI {
         if (this.hljs === null) return;
 
         const text = line.textContent;
-        const lineParent = line.parentNode as HTMLElement;
+        const lineParent = line.parentNode;
 
-        if (lineParent === null || text === null) return;
+        if (text === null || lineParent === null || !this.isElement(lineParent)) return;
 
-        const lineState = lineParent.className.indexOf('d2h-del') !== -1 ? oldLinesState : newLinesState;
+        const lineState = lineParent.classList.contains('d2h-del') ? oldLinesState : newLinesState;
 
         const language = file.getAttribute('data-lang');
         const result =
@@ -130,9 +132,9 @@ export class Diff2HtmlUI {
             : this.hljs.highlightAuto(text);
 
         if (this.instanceOfIHighlightResult(result)) {
-          if (lineParent.className.indexOf('d2h-del') !== -1) {
+          if (lineParent.classList.contains('d2h-del')) {
             oldLinesState = result.top;
-          } else if (lineParent.className.indexOf('d2h-ins') !== -1) {
+          } else if (lineParent.classList.contains('d2h-ins')) {
             newLinesState = result.top;
           } else {
             oldLinesState = result.top;
@@ -159,18 +161,15 @@ export class Diff2HtmlUI {
     const diffTable = body.getElementsByClassName('d2h-diff-table')[0];
 
     diffTable.addEventListener('mousedown', event => {
-      if (event === null || event.target === null) return;
-
-      const mouseEvent = event as MouseEvent;
-      const target = mouseEvent.target as HTMLElement;
-      const table = target.closest('.d2h-diff-table');
+      if (event === null || !this.isElement(event.target)) return;
 
+      const table = event.target.closest('.d2h-diff-table');
       if (table !== null) {
-        if (target.closest('.d2h-code-line,.d2h-code-side-line') !== null) {
+        if (event.target.closest('.d2h-code-line,.d2h-code-side-line') !== null) {
           table.classList.remove('selecting-left');
           table.classList.add('selecting-right');
           this.currentSelectionColumnId = 1;
-        } else if (target.closest('.d2h-code-linenumber,.d2h-code-side-linenumber') !== null) {
+        } else if (event.target.closest('.d2h-code-linenumber,.d2h-code-side-linenumber') !== null) {
           table.classList.remove('selecting-right');
           table.classList.add('selecting-left');
           this.currentSelectionColumnId = 0;
@@ -179,8 +178,9 @@ export class Diff2HtmlUI {
     });
 
     diffTable.addEventListener('copy', event => {
-      const clipboardEvent = event as ClipboardEvent;
-      const clipboardData = clipboardEvent.clipboardData;
+      if (!this.isClipboardEvent(event)) return;
+
+      const clipboardData = event.clipboardData;
       const text = this.getSelectedText();
 
       if (clipboardData === null || text === undefined) return;
@@ -231,4 +231,12 @@ export class Diff2HtmlUI {
 
     return text;
   }
+
+  private isElement(arg?: unknown): arg is Element {
+    return arg !== null && (arg as Element)?.classList !== undefined;
+  }
+
+  private isClipboardEvent(arg?: unknown): arg is ClipboardEvent {
+    return arg !== null && (arg as ClipboardEvent)?.clipboardData !== undefined;
+  }
 }
diff --git a/src/ui/js/highlight.js-helpers.ts b/src/ui/js/highlight.js-helpers.ts
index 24e9a38..f1fdccc 100644
--- a/src/ui/js/highlight.js-helpers.ts
+++ b/src/ui/js/highlight.js-helpers.ts
@@ -63,6 +63,10 @@ export function mergeStreams(original: NodeEvent[], highlighted: NodeEvent[], va
   let result = '';
   const nodeStack = [];
 
+  function isElement(arg?: unknown): arg is Element {
+    return arg !== null && (arg as Element)?.attributes !== undefined;
+  }
+
   function selectStream(): NodeEvent[] {
     if (!original.length || !highlighted.length) {
       return original.length ? original : highlighted;
@@ -88,9 +92,12 @@ export function mergeStreams(original: NodeEvent[], highlighted: NodeEvent[], va
   }
 
   function open(node: Node): void {
-    const htmlNode = node as HTMLElement;
-    result += `<${tag(node)} ${[].map
-      .call(htmlNode.attributes, (attr: Attr) => `${attr.nodeName}="${escape(attr.value)}"`)
+    if (!isElement(node)) {
+      throw new Error('Node is not an Element');
+    }
+
+    result += `<${tag(node)} ${Array<Attr>()
+      .map.call(node.attributes, attr => `${attr.nodeName}="${escape(attr.value)}"`)
       .join(' ')}>`;
   }
 
diff --git a/website/templates/pages/demo/demo.ts b/website/templates/pages/demo/demo.ts
index 82010ca..2739fd2 100644
--- a/website/templates/pages/demo/demo.ts
+++ b/website/templates/pages/demo/demo.ts
@@ -208,6 +208,30 @@ type Elements = {
   };
 };
 
+function isHTMLInputElement(arg?: unknown): arg is HTMLInputElement {
+  return arg !== null && (arg as HTMLInputElement)?.value !== undefined;
+}
+
+function getHTMLInputElementById(id: string): HTMLInputElement {
+  const element = document.getElementById(id);
+
+  if (!isHTMLInputElement(element)) {
+    throw new Error(`Could not find html input element with id '${id}'`);
+  }
+
+  return element;
+}
+
+function getHTMLElementById(id: string): HTMLElement {
+  const element = document.getElementById(id);
+
+  if (element === null) {
+    throw new Error(`Could not find html element with id '${id}'`);
+  }
+
+  return element;
+}
+
 document.addEventListener('DOMContentLoaded', async () => {
   // Improves browser compatibility
   require('whatwg-fetch');
@@ -226,20 +250,20 @@ document.addEventListener('DOMContentLoaded', async () => {
 
   const elements: Elements = {
     structure: {
-      diffTarget: document.getElementById('url-diff-container') as HTMLElement,
+      diffTarget: getHTMLElementById('url-diff-container'),
     },
     url: {
-      input: document.getElementById('url') as HTMLInputElement,
-      button: document.getElementById('url-btn') as HTMLElement,
+      input: getHTMLInputElementById('url'),
+      button: getHTMLElementById('url-btn'),
     },
     options: {
-      outputFormat: document.getElementById('diff-url-options-output-format') as HTMLInputElement,
-      matching: document.getElementById('diff-url-options-matching') as HTMLInputElement,
-      wordsThreshold: document.getElementById('diff-url-options-match-words-threshold') as HTMLInputElement,
-      matchingMaxComparisons: document.getElementById('diff-url-options-matching-max-comparisons') as HTMLInputElement,
+      outputFormat: getHTMLInputElementById('diff-url-options-output-format'),
+      matching: getHTMLInputElementById('diff-url-options-matching'),
+      wordsThreshold: getHTMLInputElementById('diff-url-options-match-words-threshold'),
+      matchingMaxComparisons: getHTMLInputElementById('diff-url-options-matching-max-comparisons'),
     },
     checkboxes: {
-      drawFileList: document.getElementById('diff-url-options-show-files') as HTMLInputElement,
+      drawFileList: getHTMLInputElementById('diff-url-options-show-files'),
     },
   };