diff --git a/_tools/check_circular_submodule_dependencies.ts b/_tools/check_circular_submodule_dependencies.ts
index 605c9d01eeac..ec21f21a4c91 100644
--- a/_tools/check_circular_submodule_dependencies.ts
+++ b/_tools/check_circular_submodule_dependencies.ts
@@ -169,8 +169,8 @@ const STABILITY: Record<Mod, DepState> = {
 const root = new URL("../", import.meta.url).href;
 const deps: Record<string, Dep> = {};
 
-function getSubmoduleNameFromUrl(url: string) {
-  return url.replace(root, "").split("/")[0];
+function getSubmoduleNameFromUrl(url: string): string {
+  return url.replace(root, "").split("/")[0]!;
 }
 
 async function check(
@@ -270,9 +270,9 @@ function stateToNodeStyle(state: DepState) {
 if (Deno.args.includes("--graph")) {
   console.log("digraph std_deps {");
   for (const mod of Object.keys(deps)) {
-    const info = deps[mod];
+    const info = deps[mod]!;
     console.log(`  ${formatLabel(mod)} ${stateToNodeStyle(info.state)};`);
-    for (const dep of deps[mod].set) {
+    for (const dep of info.set) {
       console.log(`  ${formatLabel(mod)} -> ${dep};`);
     }
   }
diff --git a/_tools/check_doc_imports.ts b/_tools/check_doc_imports.ts
index 64c7125a04ee..61c25168f816 100644
--- a/_tools/check_doc_imports.ts
+++ b/_tools/check_doc_imports.ts
@@ -81,11 +81,11 @@ function checkCodeBlocks(
 
     if (
       ["ts", "js", "typescript", "javascript", ""].includes(
-        language.toLocaleLowerCase(),
+        language!.toLocaleLowerCase(),
       )
     ) {
       checkImportStatements(
-        codeBlock,
+        codeBlock!,
         filePath,
         lineNumber + codeBlockLineNumber,
       );
diff --git a/_tools/check_licence.ts b/_tools/check_licence.ts
index 47b20fe7d55f..504eb32ef2a4 100644
--- a/_tools/check_licence.ts
+++ b/_tools/check_licence.ts
@@ -45,7 +45,8 @@ for await (
       console.log("Copyright header automatically added to " + path);
     }
   } else if (
-    parseInt(match[1]) !== FIRST_YEAR || parseInt(match[2]) !== CURRENT_YEAR
+    (match[1] && parseInt(match[1]) !== FIRST_YEAR) ||
+    (match[2] && parseInt(match[2]) !== CURRENT_YEAR)
   ) {
     if (CHECK) {
       console.error(`Incorrect copyright year: ${path}`);
diff --git a/_tools/convert_to_workspace.ts b/_tools/convert_to_workspace.ts
index 9a35bea8b9d5..aa0bd9f86699 100644
--- a/_tools/convert_to_workspace.ts
+++ b/_tools/convert_to_workspace.ts
@@ -94,14 +94,14 @@ for (const { specifier, dependencies } of graph.modules) {
     continue;
   }
   const from = relative(cwd, fromFileUrl(specifier)).replaceAll("\\", "/");
-  const fromPkg = from.split("/")[0];
+  const fromPkg = from.split("/")[0]!;
   for (const dep of dependencies ?? []) {
     if (dep.code) {
       const to = relative(cwd, fromFileUrl(dep.code.specifier)).replaceAll(
         "\\",
         "/",
       );
-      const toPkg = to.split("/")[0];
+      const toPkg = to.split("/")[0]!;
       if (fromPkg !== toPkg) {
         pkgDeps.get(fromPkg)!.add(toPkg);
       }
@@ -111,7 +111,7 @@ for (const { specifier, dependencies } of graph.modules) {
         "\\",
         "/",
       );
-      const toPkg = to.split("/")[0];
+      const toPkg = to.split("/")[0]!;
       if (fromPkg !== toPkg) {
         pkgDeps.get(fromPkg)!.add(toPkg);
       }
@@ -221,7 +221,7 @@ for await (const entry of walk(cwd)) {
 for (const pkg of packages) {
   const exportsList = exportsByPackage.get(pkg)!;
   let exports;
-  if (exportsList.length === 1 && exportsList[0][0] === ".") {
+  if (exportsList.length === 1 && exportsList[0]![0] === ".") {
     exports = "./mod.ts";
   } else {
     exports = Object.fromEntries(exportsList);