diff --git a/frontend/src/chart_series.ts b/frontend/src/chart_series.ts
index 34ffb4d1..bd6b48a1 100644
--- a/frontend/src/chart_series.ts
+++ b/frontend/src/chart_series.ts
@@ -40,6 +40,7 @@ export function xaxis_type_for(
     NUMERIC_X_CHART_TYPES.includes(chart_type)
   )
     return "numeric";
+  return undefined;
 }
 
 /**
diff --git a/tests/end-to-end/fixture.ts b/tests/end-to-end/fixture.ts
index c975b948..c6addb1c 100644
--- a/tests/end-to-end/fixture.ts
+++ b/tests/end-to-end/fixture.ts
@@ -1,7 +1,7 @@
 import path from "node:path";
 import { test as base, expect } from "@playwright/test";
 
-const fixturesDirectory = path.resolve(__dirname, "fixtures");
+const fixturesDirectory = path.resolve(import.meta.dirname, "fixtures");
 
 export const test = base.extend({
   page: async ({ page }, use, testInfo) => {
diff --git a/tests/end-to-end/fixtures/big-number/test.ts b/tests/end-to-end/fixtures/big-number/test.ts
index a157eb9e..8dd52dc0 100644
--- a/tests/end-to-end/fixtures/big-number/test.ts
+++ b/tests/end-to-end/fixtures/big-number/test.ts
@@ -1,4 +1,4 @@
-import { expect, test } from "../../fixture";
+import { expect, test } from "../../fixture.ts";
 
 for (const theme of ["light", "dark"]) {
   test(`colored values match their cards in the ${theme} theme`, async ({
diff --git a/tests/end-to-end/fixtures/chart/test.ts b/tests/end-to-end/fixtures/chart/test.ts
index fad452e6..1844d39a 100644
--- a/tests/end-to-end/fixtures/chart/test.ts
+++ b/tests/end-to-end/fixtures/chart/test.ts
@@ -1,4 +1,4 @@
-import { expect, type Page, test } from "../../fixture";
+import { expect, type Page, test } from "../../fixture.ts";
 
 type ChartPoint = { x: string | number | Date; y: number | null };
 
diff --git a/tests/end-to-end/fixtures/form/test.ts b/tests/end-to-end/fixtures/form/test.ts
index 7531b591..24729473 100644
--- a/tests/end-to-end/fixtures/form/test.ts
+++ b/tests/end-to-end/fixtures/form/test.ts
@@ -1,4 +1,4 @@
-import { expect, test } from "../../fixture";
+import { expect, test } from "../../fixture.ts";
 
 const fields = [
   { selector: 'input[name="modern_text"]', name: "text" },
diff --git a/tests/end-to-end/fixtures/map/test.ts b/tests/end-to-end/fixtures/map/test.ts
index f6c87566..a6565076 100644
--- a/tests/end-to-end/fixtures/map/test.ts
+++ b/tests/end-to-end/fixtures/map/test.ts
@@ -1,4 +1,4 @@
-import { expect, type Page, test } from "../../fixture";
+import { expect, type Page, test } from "../../fixture.ts";
 
 const PARIS_WITHOUT_ITS_LONGITUDE = "48.85,";
 const NOT_COORDINATES = "somewhere nice";
diff --git a/tests/end-to-end/package.json b/tests/end-to-end/package.json
index e3d19fcb..5a72f306 100644
--- a/tests/end-to-end/package.json
+++ b/tests/end-to-end/package.json
@@ -1,6 +1,7 @@
 {
   "name": "end-to-end",
   "version": "1.0.0",
+  "type": "module",
   "description": "",
   "main": "index.js",
   "scripts": {
diff --git a/tsconfig.json b/tsconfig.json
index 045c63f9..7d017ff4 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,9 +6,19 @@
     "moduleResolution": "bundler",
     "noEmit": true,
     "allowImportingTsExtensions": true,
+    "erasableSyntaxOnly": true,
+    "isolatedModules": true,
+    "verbatimModuleSyntax": true,
     "strict": true,
+    "noImplicitOverride": true,
+    "noFallthroughCasesInSwitch": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "allowUnreachableCode": false,
+    "allowUnusedLabels": false,
     "noImplicitAny": false,
-    "types": []
+    "types": [],
+    "noImplicitReturns": true
   },
   "include": ["frontend/src"]
 }