diff --git a/test/JavaScriptObfuscator.spec.ts b/test/JavaScriptObfuscator.spec.ts
index 5358d498..fdb5a288 100644
--- a/test/JavaScriptObfuscator.spec.ts
+++ b/test/JavaScriptObfuscator.spec.ts
@@ -2,7 +2,7 @@ import { JavaScriptObfuscator } from "../src/JavaScriptObfuscator";
 
 import { NO_CUSTOM_NODES_PRESET } from "../src/preset-options/NoCustomNodesPreset";
 
-let assert: any = require('chai').assert;
+const assert: Chai.AssertStatic = require('chai').assert;
 
 describe('JavaScriptObfuscator', () => {
     describe('obfuscate (sourceCode: string, customOptions?: IOptionsPreset): string', () => {
diff --git a/test/JavaScriptObfuscatorCLI.spec.ts b/test/JavaScriptObfuscatorCLI.spec.ts
index 8a87f836..309abcbe 100644
--- a/test/JavaScriptObfuscatorCLI.spec.ts
+++ b/test/JavaScriptObfuscatorCLI.spec.ts
@@ -2,7 +2,7 @@ import * as fs from 'fs';
 
 import { JavaScriptObfuscator } from "../src/JavaScriptObfuscator";
 
-let assert: any = require('chai').assert;
+const assert: Chai.AssertStatic = require('chai').assert;
 
 describe('JavaScriptObfuscatorCLI', () => {
     let fixturesDirName: string = 'test/fixtures',
diff --git a/test/NodeUtils.spec.ts b/test/NodeUtils.spec.ts
index 06769a65..f1f54ea7 100644
--- a/test/NodeUtils.spec.ts
+++ b/test/NodeUtils.spec.ts
@@ -10,7 +10,7 @@ import { NodeType } from "../src/enums/NodeType";
 
 import { NodeUtils } from '../src/NodeUtils';
 
-let assert: any = require('chai').assert;
+const assert: any = require('chai').assert;
 
 function getProgramNode (bodyNodes: INode[] = []): IProgramNode {
     return {
diff --git a/test/OptionsNormalizer.spec.ts b/test/OptionsNormalizer.spec.ts
index ae8f4e82..92c0ecae 100644
--- a/test/OptionsNormalizer.spec.ts
+++ b/test/OptionsNormalizer.spec.ts
@@ -4,7 +4,7 @@ import { OptionsNormalizer } from '../src/OptionsNormalizer';
 
 import { DEFAULT_PRESET } from '../src/preset-options/DefaultPreset';
 
-let assert: any = require('chai').assert;
+const assert: Chai.AssertStatic = require('chai').assert;
 
 describe('OptionsNormalizer', () => {
     describe('normalizeOptionsPreset (options: IOptionsPreset): IOptionsPreset', () => {
diff --git a/test/Utils.spec.ts b/test/Utils.spec.ts
index f4f5dadf..4b182c43 100644
--- a/test/Utils.spec.ts
+++ b/test/Utils.spec.ts
@@ -2,7 +2,7 @@ import { Utils } from '../src/Utils';
 
 import { JSFuck } from '../src/enums/JSFuck';
 
-let assert: any = require('chai').assert;
+const assert: Chai.AssertStatic = require('chai').assert;
 
 describe('Utils', () => {
     describe('arrayContains (array: any[], searchElement: any): boolean', () => {
diff --git a/test/dev/test.js b/test/dev/test.js
index 7736000d..10da0317 100644
--- a/test/dev/test.js
+++ b/test/dev/test.js
@@ -1,6 +1,6 @@
 'use strict';
 
-let JavaScriptObfuscator = require('../../dist/index');
+const JavaScriptObfuscator = require('../../dist/index');
 
 let obfuscatedCode = JavaScriptObfuscator.obfuscate(
     `