diff --git a/__testsUtils__/globals.d.ts b/__testsUtils__/globals.d.ts
new file mode 100644
index 0000000..700a279
--- /dev/null
+++ b/__testsUtils__/globals.d.ts
@@ -0,0 +1,8 @@
+import type { MongoMemoryServer } from 'mongodb-memory-server';
+import type { MongoClient, Db } from 'mongodb';
+
+declare global {
+  var mongoServer: MongoMemoryServer;
+  var mongoClient: MongoClient;
+  var db: Db;
+}
diff --git a/__testsUtils__/mongoValidation.ts b/__testsUtils__/mongoValidation.ts
index 3d243bd..9c6d500 100644
--- a/__testsUtils__/mongoValidation.ts
+++ b/__testsUtils__/mongoValidation.ts
@@ -13,7 +13,7 @@ async function setValidationLevel(
   const errors: Record<string, any> = {};
   for (const collection of collections) {
     try {
-      await global.db.runCommand({
+      await global.db.command({
         collMod: collection,
         validationLevel: validationLevel,
       });
diff --git a/__tests__/MongoBulkDataMigration.update.test.ts b/__tests__/MongoBulkDataMigration.update.test.ts
index 10bfc96..82b9bf0 100644
--- a/__tests__/MongoBulkDataMigration.update.test.ts
+++ b/__tests__/MongoBulkDataMigration.update.test.ts
@@ -226,7 +226,7 @@ describe('MongoBulkDataMigration', () => {
 
     describe('Bulk splitting', () => {
       const END_OF_BULK_LOG = 'Documents migration is successful';
-      let update: UpdateFilter<{ value: number }>;
+      let update: (arg: { value: number }) => UpdateFilter<{ value: number }>;
       beforeEach(async () => {
         await collection.insertMany(
           Array.from({ length: 100 }, (_, i) => ({ value: i + 1 })),
diff --git a/src/MongoBulkDataMigration.ts b/src/MongoBulkDataMigration.ts
index 50e03e0..1a55f89 100644
--- a/src/MongoBulkDataMigration.ts
+++ b/src/MongoBulkDataMigration.ts
@@ -441,7 +441,12 @@ export default class MongoBulkDataMigration<
     try {
       await migrationCollection.drop();
     } catch (err) {
-      if (err?.message === 'ns not found') {
+      if (
+        err &&
+        typeof err === 'object' &&
+        'message' in err &&
+        err.message === 'ns not found'
+      ) {
         return;
       }
       throw err;
diff --git a/src/lib/AbstractBulkOperationResults.ts b/src/lib/AbstractBulkOperationResults.ts
index 0f8868a..35bfb79 100644
--- a/src/lib/AbstractBulkOperationResults.ts
+++ b/src/lib/AbstractBulkOperationResults.ts
@@ -54,9 +54,9 @@ export abstract class AbstractBulkOperationResults<TSchema extends Document> {
     this.collection = collection;
     this.totalOperations = totalOperations;
     this.logger = logger;
+    this.bulk = this.collection.initializeUnorderedBulkOp();
     this.totalBulkOps = 0;
     this.results = _.cloneDeep(INITIAL_BULK_INFOS);
-    this.initialize();
   }
 
   abstract logExecutionStatus(executionResults: BulkOperationResult): this;
@@ -78,12 +78,6 @@ export abstract class AbstractBulkOperationResults<TSchema extends Document> {
     );
   }
 
-  private initialize(): this {
-    this.bulk = this.collection.initializeUnorderedBulkOp();
-    this.totalBulkOps = 0;
-    return this;
-  }
-
   public async execute(continueOnBulkWriteError = false): Promise<this> {
     if (this.totalBulkOps === 0) {
       return this;
@@ -117,7 +111,10 @@ export abstract class AbstractBulkOperationResults<TSchema extends Document> {
     this.mergeResults(resultPartial);
     this.logExecutionStatus(resultPartial);
 
-    return this.initialize();
+    this.bulk = this.collection.initializeUnorderedBulkOp();
+    this.totalBulkOps = 0;
+
+    return this;
   }
 
   private mergeResults(resultB: BulkOperationResult): this {
diff --git a/src/lib/computeRollbackQuery.ts b/src/lib/computeRollbackQuery.ts
index 1278fb7..275b96c 100644
--- a/src/lib/computeRollbackQuery.ts
+++ b/src/lib/computeRollbackQuery.ts
@@ -43,7 +43,7 @@ function computeArrayFilterAndUnsetWithPositionals(
   updateQuery: any,
   backup: any,
 ): { arrayFilters: Document[]; unsetAdditionalPositional: any } {
-  const unsetAdditionalPositional = {};
+  const unsetAdditionalPositional: Record<string, number> = {};
   const filteredObject = Object.keys(flattenDocument(backup));
   const arrayFilters: Document[] = [];
   const update = Object.keys(flattenDocument(updateQuery.$set ?? {}));
diff --git a/src/testUtils/doRollbackAndAssertForInitialState.ts b/src/testUtils/doRollbackAndAssertForInitialState.ts
index 2ecb6d7..a7eb65a 100644
--- a/src/testUtils/doRollbackAndAssertForInitialState.ts
+++ b/src/testUtils/doRollbackAndAssertForInitialState.ts
@@ -2,7 +2,7 @@ import _ from 'lodash';
 import MongoBulkDataMigration from '../MongoBulkDataMigration';
 import { BulkOperationResult } from '../lib/AbstractBulkOperationResults';
 
-let globalExpect = null;
+let globalExpect: ((value: unknown) => unknown) | null = null;
 
 export function setGlobalExpect(expect: any) {
   globalExpect = expect;
@@ -59,7 +59,7 @@ function assertDeepHaveMembers(
   effectiveDocs: any,
   expectedDocs: any,
 ) {
-  const compareObjectId = (objA, objB) => {
+  const compareObjectId = (objA: string, objB: string) => {
     const sortedIds = [objA, objB].sort();
     return sortedIds[0] === objA ? -1 : 1;
   };
diff --git a/src/tools/arrayFilters.ts b/src/tools/arrayFilters.ts
index 93579b1..c7b90c3 100644
--- a/src/tools/arrayFilters.ts
+++ b/src/tools/arrayFilters.ts
@@ -1,3 +1,5 @@
+import type { Document } from 'mongodb';
+
 /**
  * Spots "positional operator" path in string for array filters
  * Example: xxx.$[element_name].yyy
@@ -72,7 +74,7 @@ export function hasPathMatchingPositionalOperators(
  */
 export function buildArrayFiltersOptionToUnset(path: string) {
   let positionalElementNameMatch;
-  const arrayFilters = [];
+  const arrayFilters: Document[] = [];
   do {
     positionalElementNameMatch = ARRAY_FILTER_OPERATION_PATTERN.exec(path);
     if (positionalElementNameMatch) {
@@ -115,7 +117,7 @@ function _buildSubPathWithPositionalElementAndNextChild(
     `\\$\\[${positionalElementName}]\\.(\\w+)(?:\\.|$)`,
   );
 
-  const nextChildName = matchRegexp.exec(completePath)[1];
+  const nextChildName = matchRegexp.exec(completePath)?.[1];
 
   return `${positionalElementName}.${nextChildName}`;
 }
diff --git a/tsconfig.json b/tsconfig.json
index c6b005d..dc3c13c 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,5 +1,18 @@
 {
   "compilerOptions": {
+    /* Type Checking */
+    "allowUnreachableCode": false,
+    "allowUnusedLabels": false,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitAny": true,
+    "noImplicitReturns": true,
+    "noImplicitThis": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "skipLibCheck": true,
+    "strict": true,
+    "useUnknownInCatchVariables": true,
+
     "target": "es2022",
     "module": "commonjs",
     "lib": ["ES2022"],
@@ -10,14 +23,7 @@
     "allowSyntheticDefaultImports": true,
     "alwaysStrict": true,
     "sourceMap": true,
-    "forceConsistentCasingInFileNames": true,
-    "noFallthroughCasesInSwitch": true,
-    "noImplicitReturns": true,
-    "noUnusedLocals": true,
-    "noUnusedParameters": true,
-    "noImplicitAny": false,
-    "noImplicitThis": false,
-    "strictNullChecks": false
+    "forceConsistentCasingInFileNames": true
   },
   "include": [
     "jest.config.ts",