diff --git a/.eslintrc.cjs b/.eslintrc.cjs
index 2be1587c..f21ee3c8 100644
--- a/.eslintrc.cjs
+++ b/.eslintrc.cjs
@@ -5,6 +5,7 @@ module.exports = {
     'eslint:recommended',
     'plugin:@typescript-eslint/recommended',
     'plugin:@typescript-eslint/recommended-requiring-type-checking',
+    'plugin:@typescript-eslint/strict',
     'prettier'
   ],
   plugins: ['svelte3', 'tailwindcss', '@typescript-eslint', 'es', 'vitest'],
@@ -19,7 +20,10 @@ module.exports = {
     'package.json',
     'tsconfig.json'
   ],
-  overrides: [{ files: ['*.svelte'], processor: 'svelte3/svelte3' }],
+  overrides: [
+    { files: ['*.svelte'], processor: 'svelte3/svelte3' }
+    // { files: ['*.ts'], extends: ['plugin:@typescript-eslint/recommended-requiring-type-checking'] }
+  ],
   settings: {
     'svelte3/typescript': () => require('typescript')
   },
diff --git a/.vscode/settings.json b/.vscode/settings.json
index ff638551..28b8758f 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -1,6 +1,6 @@
 {
   "editor.formatOnSave": true,
-  "cSpell.words": ["asyncable", "mindmap", "pako", "Serde", "serdes"],
+  "cSpell.words": ["asyncable", "mindmap", "pako", "Serde", "serdes", "tailwindcss"],
   "vitest.commandLine": "yarn test:unit",
   "vitest.enable": true,
   "testing.autoRun.mode": "rerun",
diff --git a/src/global.d.ts b/src/global.d.ts
index 81ca3d20..95ecd6a0 100644
--- a/src/global.d.ts
+++ b/src/global.d.ts
@@ -9,7 +9,8 @@ declare global {
   namespace jest {
     interface Matchers<R = void>
       // eslint-disable-next-line @typescript-eslint/ban-ts-comment
-      // @ts-ignore
+      // @ts-expect-error
+      // eslint-disable-next-line no-undef
       extends TestingLibraryMatchers<typeof expect.stringContaining, R> {}
   }
 }
diff --git a/src/lib/components/Actions.svelte b/src/lib/components/Actions.svelte
index 6121fd51..166a58cd 100644
--- a/src/lib/components/Actions.svelte
+++ b/src/lib/components/Actions.svelte
@@ -15,8 +15,8 @@
     `mermaid-diagram-${moment().format('YYYY-MM-DD-HHmmss')}.${ext}`;
 
   const getBase64SVG = (svg?: HTMLElement, width?: number, height?: number): string => {
-    svg?.setAttribute('height', `${height}px`);
-    svg?.setAttribute('width', `${width}px`); // Workaround https://stackoverflow.com/questions/28690643/firefox-error-rendering-an-svg-image-to-html5-canvas-with-drawimage
+    height && svg?.setAttribute('height', `${height}px`);
+    width && svg?.setAttribute('width', `${width}px`); // Workaround https://stackoverflow.com/questions/28690643/firefox-error-rendering-an-svg-image-to-html5-canvas-with-drawimage
     if (!svg) {
       svg = getSvgEl();
     }
@@ -28,7 +28,10 @@
 
   const exportImage = (event: Event, exporter: Exporter) => {
     const canvas: HTMLCanvasElement = document.createElement('canvas');
-    const svg: HTMLElement = document.querySelector('#container svg');
+    const svg: HTMLElement | null = document.querySelector('#container svg');
+    if (!svg) {
+      throw new Error('svg not found');
+    }
     const box: DOMRect = svg.getBoundingClientRect();
     canvas.width = box.width;
     canvas.height = box.height;
@@ -43,6 +46,9 @@
     }
 
     const context = canvas.getContext('2d');
+    if (!context) {
+      throw new Error('context not found');
+    }
     context.fillStyle = 'white';
     context.fillRect(0, 0, canvas.width, canvas.height);
 
@@ -56,12 +62,12 @@
 
   const getSvgEl = () => {
     const svgEl: HTMLElement = document
-      .querySelector('#container svg')
+      .querySelector('#container svg')!
       .cloneNode(true) as HTMLElement;
     svgEl.setAttribute('xmlns:xlink', 'http://www.w3.org/1999/xlink');
     const fontAwesomeCdnUrl = Array.from(document.head.getElementsByTagName('link'))
       .map((l) => l.href)
-      .find((h) => h && h.includes('font-awesome'));
+      .find((h) => h.includes('font-awesome'));
     if (fontAwesomeCdnUrl == null) {
       return svgEl;
     }
@@ -99,10 +105,10 @@
       context.drawImage(image, 0, 0, canvas.width, canvas.height);
       canvas.toBlob((blob) => {
         try {
-          // @ts-ignore: https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/1004/files
+          if (!blob) {
+            throw new Error('blob is empty');
+          }
           void navigator.clipboard.write([
-            /* eslint-disable no-undef */
-            // @ts-ignore: https://github.com/microsoft/TypeScript/issues/43821
             new ClipboardItem({
               [blob.type]: blob
             })
@@ -142,7 +148,7 @@
   let gistURL = '';
   stateStore.subscribe(({ loader }) => {
     if (loader?.type === 'gist') {
-      // @ts-ignore Gist will have url
+      // @ts-expect-error Gist will have url
       gistURL = loader.config.url;
     }
   });
diff --git a/src/lib/components/Editor.svelte b/src/lib/components/Editor.svelte
index 19011ed4..5525afe9 100644
--- a/src/lib/components/Editor.svelte
+++ b/src/lib/components/Editor.svelte
@@ -8,9 +8,9 @@
   import initEditor from 'monaco-mermaid';
   import { logEvent } from '$lib/util/stats';
 
-  let divEl: HTMLDivElement = null;
-  let editor: monaco.editor.IStandaloneCodeEditor;
-  let Monaco: typeof monaco;
+  let divEl: HTMLDivElement | undefined = undefined;
+  let editor: monaco.editor.IStandaloneCodeEditor | undefined;
+  let Monaco: typeof monaco | undefined;
   let editorOptions: monaco.editor.IStandaloneEditorConstructionOptions = {
     minimap: {
       enabled: false
@@ -22,7 +22,7 @@
 
   stateStore.subscribe(({ errorMarkers, editorMode, code, mermaid }) => {
     console.log('editor store subscription', { code, mermaid });
-    if (!editor) {
+    if (!editor || !Monaco) {
       return;
     }
 
@@ -36,12 +36,17 @@
 
     // Update editor mode if it's different
     const language = editorMode === 'code' ? 'mermaid' : 'json';
-    if (editor.getModel().getLanguageId() !== language) {
-      Monaco?.editor.setModelLanguage(editor.getModel(), language);
+    const model = editor.getModel();
+    if (!model) {
+      console.error("editor model doesn't exist");
+      return;
+    }
+    if (model.getLanguageId() !== language) {
+      Monaco.editor.setModelLanguage(model, language);
     }
 
     // Display/clear errors
-    Monaco?.editor.setModelMarkers(editor.getModel(), 'mermaid', errorMarkers);
+    Monaco.editor.setModelMarkers(model, 'mermaid', errorMarkers);
   });
 
   themeStore.subscribe(({ isDark }) => {
@@ -62,7 +67,7 @@
     // errorDebug();
     let i = 0;
     while (i++ < 500) {
-      // @ts-ignore : This is a hack to handle a svelte-kit error when importing monaco.
+      // @ts-expect-error : This is a hack to handle a svelte-kit error when importing monaco.
       Monaco = window.monaco;
       if (Monaco !== undefined) {
         return;
@@ -74,14 +79,20 @@
 
   onMount(async () => {
     await loadMonaco(); // Fix https://github.com/mermaid-js/mermaid-live-editor/issues/175
+    if (!Monaco) {
+      throw new Error('Monaco failed to load');
+    }
+    if (!divEl) {
+      throw new Error('divEl is undefined');
+    }
     // eslint-disable-next-line @typescript-eslint/no-unsafe-call
     initEditor(Monaco);
     errorDebug(100);
     editor = Monaco.editor.create(divEl, editorOptions);
     editor.onDidChangeModelContent(({ isFlush, changes }) => {
-      const newText = editor.getValue();
+      const newText = editor?.getValue();
       console.log('editor onDidChangeModelContent', { text, newText, isFlush, changes });
-      if (text === newText || isFlush) {
+      if (!newText || text === newText || isFlush) {
         return;
       }
       text = newText;
@@ -98,19 +109,21 @@
         });
       }
     });
-    Monaco?.editor.setTheme($themeStore.isDark ? 'mermaid-dark' : 'mermaid');
+    Monaco.editor.setTheme($themeStore.isDark ? 'mermaid-dark' : 'mermaid');
     const resizeObserver = new ResizeObserver((entries) => {
-      editor.layout({
+      editor!.layout({
         height: entries[0].contentRect.height,
         width: entries[0].contentRect.width
       });
     });
 
-    resizeObserver.observe(divEl.parentElement);
+    if (divEl.parentElement) {
+      resizeObserver.observe(divEl.parentElement);
+    }
     console.log(`editor mounted`);
     return () => {
       console.log(`editor disposed`);
-      editor.dispose();
+      editor?.dispose();
     };
   });
 </script>
diff --git a/src/lib/components/Preset.svelte b/src/lib/components/Preset.svelte
index 0eae7dc5..ad3831ee 100644
--- a/src/lib/components/Preset.svelte
+++ b/src/lib/components/Preset.svelte
@@ -114,8 +114,8 @@
   };
 
   // Adding in this array will add an icon to the preset menu
-  const newDiagrams: Array<SampleTypes> = ['Mindmap'];
-  const diagramOrder: Array<SampleTypes> = [
+  const newDiagrams: SampleTypes[] = ['Mindmap'];
+  const diagramOrder: SampleTypes[] = [
     'Sequence',
     'Flow',
     'Class',
diff --git a/src/lib/components/Theme.svelte b/src/lib/components/Theme.svelte
index 6f456a68..4226159d 100644
--- a/src/lib/components/Theme.svelte
+++ b/src/lib/components/Theme.svelte
@@ -25,7 +25,7 @@
     '🧛‍♂️ dracula'
   ];
 
-  function checkTheme(theme: string) {
+  function checkTheme(theme: string): boolean {
     return theme.includes($themeStore.theme);
   }
 </script>
diff --git a/src/lib/components/View.svelte b/src/lib/components/View.svelte
index 46f0cf0d..28fa976f 100644
--- a/src/lib/components/View.svelte
+++ b/src/lib/components/View.svelte
@@ -17,9 +17,12 @@
   let hide = false;
   let manualUpdate = true;
   let panZoomEnabled = $stateStore.panZoom;
-  let pzoom: typeof panzoom;
+  let pzoom: typeof panzoom | undefined;
 
   const handlePanZoomChange = () => {
+    if (!pzoom) {
+      return;
+    }
     const pan = pzoom.getPan();
     const zoom = pzoom.getZoom();
     updateCodeStore({ pan, zoom });
@@ -35,6 +38,9 @@
     pzoom = undefined;
     void Promise.resolve().then(() => {
       const graphDiv = document.getElementById('graph-div');
+      if (!graphDiv) {
+        return;
+      }
       pzoom = panzoom(graphDiv, {
         onPan: handlePanZoomChange,
         onZoom: handlePanZoomChange,
@@ -71,7 +77,7 @@
         code = state.code;
         config = state.mermaid;
         panZoomEnabled = state.panZoom;
-        const scroll = view.parentElement.scrollTop;
+        const scroll = view.parentElement!.scrollTop;
         delete container.dataset.processed;
         await renderDiagram(
           Object.assign({}, JSON.parse(state.mermaid)) as MermaidConfig,
@@ -83,6 +89,9 @@
               container.innerHTML = svgCode;
               // console.log(container.innerHTML);
               const graphDiv = document.getElementById('graph-div');
+              if (!graphDiv) {
+                throw new Error('graph-div not found');
+              }
               graphDiv.setAttribute('height', '100%');
               graphDiv.style.maxWidth = '100%';
               if (bindFunctions) {
@@ -91,7 +100,7 @@
             }
           }
         );
-        view.parentElement.scrollTop = scroll;
+        view.parentElement!.scrollTop = scroll;
         error = false;
       } else if (manualUpdate) {
         manualUpdate = false;
diff --git a/src/lib/types.d.ts b/src/lib/types.d.ts
index 30f94b9c..33a3fc69 100644
--- a/src/lib/types.d.ts
+++ b/src/lib/types.d.ts
@@ -73,12 +73,13 @@ export type HistoryEntry = { id: string; state: State; time: number; url?: strin
     }
 );
 
-export interface DocConfig {
-  [key: string]: {
+export type DocConfig = Record<
+  string,
+  {
     code: string;
     config?: string;
-  };
-}
+  }
+>;
 
 export type EditorMode = 'code' | 'config';
 
diff --git a/src/lib/util/fileLoaders/loader.ts b/src/lib/util/fileLoaders/loader.ts
index e40a988f..596d02a2 100644
--- a/src/lib/util/fileLoaders/loader.ts
+++ b/src/lib/util/fileLoaders/loader.ts
@@ -8,10 +8,11 @@ const loaders: Record<string, Loader> = {
 export const loadDataFromUrl = async (): Promise<void> => {
   const searchParams = new URLSearchParams(window.location.search);
   let state: Partial<State> = defaultState;
-  let code: string, config: string;
+  let code: string | undefined = undefined;
+  let config: string | undefined = undefined;
   let loaded = false;
-  const codeURL: string = searchParams.get('code');
-  const configURL: string = searchParams.get('config');
+  const codeURL: string | undefined = searchParams.get('code') ?? undefined;
+  const configURL: string | undefined = searchParams.get('config') ?? undefined;
 
   if (codeURL) {
     code = await (await fetch(codeURL)).text();
@@ -23,9 +24,6 @@ export const loadDataFromUrl = async (): Promise<void> => {
     config = defaultState.mermaid;
   }
   if (!code) {
-    // eslint-disable-next-line @typescript-eslint/ban-ts-comment
-    // @ts-ignore
-    // eslint-disable-next-line @typescript-eslint/no-unsafe-call
     for (const [key, value] of searchParams.entries()) {
       if (key in loaders) {
         try {
@@ -38,6 +36,9 @@ export const loadDataFromUrl = async (): Promise<void> => {
       }
     }
   } else {
+    if (!codeURL) {
+      throw new Error('Code URL is not defined');
+    }
     state = {
       code,
       mermaid: config,
diff --git a/src/lib/util/migrations.ts b/src/lib/util/migrations.ts
index ba6a7c53..7851270b 100644
--- a/src/lib/util/migrations.ts
+++ b/src/lib/util/migrations.ts
@@ -7,7 +7,7 @@ interface MigrationState {
   version: number;
 }
 
-const migrations: { [key: string]: () => void } = {
+const migrations: Record<string, () => void> = {
   injectHistoryIDs
 };
 
diff --git a/src/lib/util/persist.ts b/src/lib/util/persist.ts
index 3beb4704..584a0f76 100644
--- a/src/lib/util/persist.ts
+++ b/src/lib/util/persist.ts
@@ -39,16 +39,16 @@ let noWarnings = false;
 /**
  * List of storages where the warning have already been displayed.
  */
-const alreadyWarnFor: Array<string> = [];
+const alreadyWarnFor: string[] = [];
 
 /**
  * Add a log to indicate that the requested Storage have not been found.
  * @param {string} storageName
  */
 const warnStorageNotFound = (storageName: string) => {
-  const isProduction = typeof process !== 'undefined' && process.env?.NODE_ENV === 'production';
+  const isProduction = typeof process !== 'undefined' && process.env.NODE_ENV === 'production';
 
-  if (!noWarnings && alreadyWarnFor.indexOf(storageName) === -1 && !isProduction) {
+  if (!noWarnings && !alreadyWarnFor.includes(storageName) && !isProduction) {
     let message = `Unable to find the ${storageName}. No data will be persisted.`;
     if (typeof window === 'undefined') {
       message +=
@@ -60,17 +60,8 @@ const warnStorageNotFound = (storageName: string) => {
   }
 };
 
-const allowedClasses = [];
-/**
- * Add a class to the allowed list of classes to be serialized
- * @param classDef The class to add to the list
- */
-export const addSerializableClass = (classDef: () => unknown): void => {
-  allowedClasses.push(classDef);
-};
-
 const serialize = (value: unknown): string => ESSerializer.serialize(value);
-const deserialize = (value: string): unknown => {
+const deserialize = (value?: string | null): unknown => {
   // @TODO: to remove in the next major
   if (value === 'undefined') {
     return undefined;
@@ -78,7 +69,7 @@ const deserialize = (value: string): unknown => {
 
   if (value !== null && value !== undefined) {
     try {
-      return ESSerializer.deserialize(value, allowedClasses);
+      return ESSerializer.deserialize(value);
     } catch (e) {
       // Do nothing
       // use the value "as is"
@@ -184,7 +175,7 @@ function getBrowserStorage(
   browserStorage: Storage,
   listenExternalChanges = false
 ): SelfUpdateStorageInterface<any> {
-  const listeners: Array<{ key: string; listener: (newValue: any) => void }> = [];
+  const listeners: { key: string; listener: (newValue: any) => void }[] = [];
   const listenerFunction = (event: StorageEvent) => {
     const eventKey = event.key;
     if (event.storageArea === browserStorage) {
@@ -196,12 +187,14 @@ function getBrowserStorage(
     }
   };
   const connect = () => {
-    if (listenExternalChanges && typeof window !== 'undefined' && window?.addEventListener) {
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+    if (listenExternalChanges && typeof window !== 'undefined' && window.addEventListener) {
       window.addEventListener('storage', listenerFunction);
     }
   };
   const disconnect = () => {
-    if (listenExternalChanges && typeof window !== 'undefined' && window?.removeEventListener) {
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+    if (listenExternalChanges && typeof window !== 'undefined' && window.removeEventListener) {
       window.removeEventListener('storage', listenerFunction);
     }
   };
@@ -240,7 +233,8 @@ function getBrowserStorage(
  * @param listenExternalChanges - Update the store if the localStorage is updated from another page
  */
 export function localStorage<T>(listenExternalChanges = false): StorageInterface<T> {
-  if (typeof window !== 'undefined' && window?.localStorage) {
+  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+  if (typeof window !== 'undefined' && window.localStorage) {
     return getBrowserStorage(window.localStorage, listenExternalChanges);
   }
   warnStorageNotFound('window.localStorage');
diff --git a/src/lib/util/state.ts b/src/lib/util/state.ts
index 4d0387bd..dd298275 100644
--- a/src/lib/util/state.ts
+++ b/src/lib/util/state.ts
@@ -111,7 +111,7 @@ export const loadState = (data: string): void => {
   console.log(`Loading '${data}'`);
   try {
     state = deserializeState(data);
-    const mermaidConfig: { [key: string]: string } =
+    const mermaidConfig: Record<string, string> =
       typeof state.mermaid === 'string' ? JSON.parse(state.mermaid) : state.mermaid;
     if (
       mermaidConfig.securityLevel &&
diff --git a/src/lib/util/stats.ts b/src/lib/util/stats.ts
index b86405ce..c349088d 100644
--- a/src/lib/util/stats.ts
+++ b/src/lib/util/stats.ts
@@ -1,6 +1,6 @@
 import { browser } from '$app/environment';
 import type { AnalyticsInstance } from 'analytics';
-export let analytics: AnalyticsInstance;
+export let analytics: AnalyticsInstance | undefined;
 
 export const initAnalytics = async (): Promise<void> => {
   if (browser && !analytics) {
@@ -27,7 +27,7 @@ export const initAnalytics = async (): Promise<void> => {
   }
 };
 
-export const detectType = (text: string): string => {
+export const detectType = (text: string): string | undefined => {
   const possibleDiagramTypes = [
     'classDiagram',
     'erDiagram',
@@ -49,7 +49,7 @@ export const detectType = (text: string): string => {
 };
 
 export const countLines = (code: string): number => {
-  return (code.match(/\n/g) || '').length + 1;
+  return (code.match(/\n/g)?.length ?? 0) + 1;
 };
 
 export const saveStatistics = (graph: string): void => {
@@ -80,19 +80,20 @@ const delaysPerEvent = {
   themeChange: defaultDelay
 };
 export type AnalyticsEvent = keyof typeof delaysPerEvent;
-const timeouts: Record<string, number> = {};
+const timeouts: Map<string, number> = new Map<string, number>();
 // manual debounce to reduce the number of events sent to analytics
 export const logEvent = (name: AnalyticsEvent, data?: unknown): void => {
   if (!analytics) {
     return;
   }
   const key = data ? JSON.stringify({ name, data }) : name;
-  if (timeouts[key] === undefined) {
+  if (!timeouts.has(key)) {
     void analytics.track(name, data);
   } else {
-    clearTimeout(timeouts[key]);
+    clearTimeout(timeouts.get(key));
   }
-  timeouts[key] = window.setTimeout(() => {
-    delete timeouts[key];
-  }, delaysPerEvent[name]);
+  timeouts.set(
+    key,
+    window.setTimeout(() => timeouts.delete(key), delaysPerEvent[name])
+  );
 };
diff --git a/src/lib/util/util.ts b/src/lib/util/util.ts
index 1f8588e0..c762bc03 100644
--- a/src/lib/util/util.ts
+++ b/src/lib/util/util.ts
@@ -23,10 +23,10 @@ export const initHandler = async (): Promise<void> => {
   syncDiagram();
   initURLSubscription();
   await initAnalytics();
-  await analytics?.page();
+  await analytics.page();
 };
 
-export const isMac = navigator.platform.toUpperCase().indexOf('MAC') >= 0;
+export const isMac = navigator.platform.toUpperCase().includes('MAC');
 export const cmdKey = isMac ? 'Cmd' : 'Ctrl';
 
 let count = 0;
diff --git a/tsconfig.json b/tsconfig.json
index 38b29b7f..91263b2e 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -12,6 +12,7 @@
   "compilerOptions": {
     "resolveJsonModule": true,
     "allowSyntheticDefaultImports": true,
+    "strictNullChecks": true,
     "types": ["vitest/importMeta"]
   },
   "extends": "./.svelte-kit/tsconfig.json"