diff --git a/lib/chai/assertion.js b/lib/chai/assertion.js
index eb53fc6b..33458940 100644
--- a/lib/chai/assertion.js
+++ b/lib/chai/assertion.js
@@ -5,15 +5,12 @@
  * MIT Licensed
  */
 
-import {config} from './config.js';
-import {AssertionError} from 'assertion-error';
-import * as util from './utils/index.js';
+import { config } from "./config.js";
+import { AssertionError } from "assertion-error";
+import * as util from "./utils/index.js";
 
 /**
- * Assertion Constructor
- *
  * Creates object for chaining.
- *
  * `Assertion` objects contain metadata in the form of flags. Three flags can
  * be assigned during instantiation by passing arguments to this constructor:
  *
@@ -42,139 +39,161 @@ import * as util from './utils/index.js';
  *
  * - `eql`: This flag contains the deepEqual function to be used by the assertion.
  *
- * @param {unknown} obj target of the assertion
- * @param {string} msg (optional) custom error message
- * @param {Function} ssfi (optional) starting point for removing stack frames
- * @param {boolean} lockSsfi (optional) whether or not the ssfi flag is locked
+ * @param {unknown} ?obj target of the assertion
+ * @param {string} ?msg (optional) custom error message
+ * @param {Function} ?ssfi (optional) starting point for removing stack frames
+ * @param {boolean} ?lockSsfi (optional) whether or not the ssfi flag is locked
  * @returns {unknown}
- * @private
  */
-export function Assertion(obj, msg, ssfi, lockSsfi) {
-  util.flag(this, 'ssfi', ssfi || Assertion);
-  util.flag(this, 'lockSsfi', lockSsfi);
-  util.flag(this, 'object', obj);
-  util.flag(this, 'message', msg);
-  util.flag(this, 'eql', config.deepEqual || util.eql);
-
-  return util.proxify(this);
-}
+export class Assertion {
+  constructor(obj, msg, ssfi, lockSsfi) {
+    util.flag(this, "ssfi", ssfi || Assertion);
+    util.flag(this, "lockSsfi", lockSsfi);
+    util.flag(this, "object", obj);
+    util.flag(this, "message", msg);
+    util.flag(this, "eql", config.deepEqual || util.eql);
+
+    return util.proxify(this);
+  }
 
-Object.defineProperty(Assertion, 'includeStack', {
-  get: function () {
+  /** @returns {boolean} */
+  static get includeStack() {
     console.warn(
-      'Assertion.includeStack is deprecated, use chai.config.includeStack instead.'
+      "Assertion.includeStack is deprecated, use chai.config.includeStack instead.",
     );
     return config.includeStack;
-  },
-  set: function (value) {
+  }
+
+  /** @param {boolean} value */
+  static set includeStack(value) {
     console.warn(
-      'Assertion.includeStack is deprecated, use chai.config.includeStack instead.'
+      "Assertion.includeStack is deprecated, use chai.config.includeStack instead.",
     );
     config.includeStack = value;
   }
-});
 
-Object.defineProperty(Assertion, 'showDiff', {
-  get: function () {
+  /** @returns {boolean} */
+  static get showDiff() {
     console.warn(
-      'Assertion.showDiff is deprecated, use chai.config.showDiff instead.'
+      "Assertion.showDiff is deprecated, use chai.config.showDiff instead.",
     );
     return config.showDiff;
-  },
-  set: function (value) {
+  }
+
+  /** @param {boolean} value */
+  static set showDiff(value) {
     console.warn(
-      'Assertion.showDiff is deprecated, use chai.config.showDiff instead.'
+      "Assertion.showDiff is deprecated, use chai.config.showDiff instead.",
     );
     config.showDiff = value;
   }
-});
-
-Assertion.addProperty = function (name, fn) {
-  util.addProperty(this.prototype, name, fn);
-};
 
-Assertion.addMethod = function (name, fn) {
-  util.addMethod(this.prototype, name, fn);
-};
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   */
+  static addProperty(name, fn) {
+    util.addProperty(this.prototype, name, fn);
+  }
 
-Assertion.addChainableMethod = function (name, fn, chainingBehavior) {
-  util.addChainableMethod(this.prototype, name, fn, chainingBehavior);
-};
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   */
+  static addMethod(name, fn) {
+    util.addMethod(this.prototype, name, fn);
+  }
 
-Assertion.overwriteProperty = function (name, fn) {
-  util.overwriteProperty(this.prototype, name, fn);
-};
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   * @param {Function} chainingBehavior
+   */
+  static addChainableMethod(name, fn, chainingBehavior) {
+    util.addChainableMethod(this.prototype, name, fn, chainingBehavior);
+  }
 
-Assertion.overwriteMethod = function (name, fn) {
-  util.overwriteMethod(this.prototype, name, fn);
-};
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   */
+  static overwriteProperty(name, fn) {
+    util.overwriteProperty(this.prototype, name, fn);
+  }
 
-Assertion.overwriteChainableMethod = function (name, fn, chainingBehavior) {
-  util.overwriteChainableMethod(this.prototype, name, fn, chainingBehavior);
-};
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   */
+  static overwriteMethod(name, fn) {
+    util.overwriteMethod(this.prototype, name, fn);
+  }
 
-/**
- * ### .assert(expression, message, negateMessage, expected, actual, showDiff)
- *
- * Executes an expression and check expectations. Throws AssertionError for reporting if test doesn't pass.
- *
- * @name assert
- * @param {unknown} expression to be tested
- * @param {string | Function} message or function that returns message to display if expression fails
- * @param {string | Function} negatedMessage or function that returns negatedMessage to display if negated expression fails
- * @param {unknown} expected value (remember to check for negation)
- * @param {unknown} actual (optional) will default to `this.obj`
- * @param {boolean} showDiff (optional) when set to `true`, assert will display a diff in addition to the message if expression fails
- * @private
- */
+  /**
+   * @param {string} name
+   * @param {Function} fn
+   * @param {Function} chainingBehavior
+   */
+  static overwriteChainableMethod(name, fn, chainingBehavior) {
+    util.overwriteChainableMethod(this.prototype, name, fn, chainingBehavior);
+  }
 
-Assertion.prototype.assert = function (
-  expr,
-  msg,
-  negateMsg,
-  expected,
-  _actual,
-  showDiff
-) {
-  let ok = util.test(this, arguments);
-  if (false !== showDiff) showDiff = true;
-  if (undefined === expected && undefined === _actual) showDiff = false;
-  if (true !== config.showDiff) showDiff = false;
-
-  if (!ok) {
-    msg = util.getMessage(this, arguments);
-    let actual = util.getActual(this, arguments);
-    let assertionErrorObjectProperties = {
-      actual: actual,
-      expected: expected,
-      showDiff: showDiff
-    };
-
-    let operator = util.getOperator(this, arguments);
-    if (operator) {
-      assertionErrorObjectProperties.operator = operator;
+  /**
+   * ### .assert(expression, message, negateMessage, expected, actual, showDiff)
+   *
+   * Executes an expression and check expectations. Throws AssertionError for reporting if test doesn't pass.
+   *
+   * @name assert
+   * @param {unknown} _expr to be tested
+   * @param {string | Function} msg or function that returns message to display if expression fails
+   * @param {string | Function} _negateMsg or function that returns negatedMessage to display if negated expression fails
+   * @param {unknown} expected value (remember to check for negation)
+   * @param {unknown} _actual (optional) will default to `this.obj`
+   * @param {boolean} showDiff (optional) when set to `true`, assert will display a diff in addition to the message if expression fails
+   */
+  assert(_expr, msg, _negateMsg, expected, _actual, showDiff) {
+    var ok = util.test(this, arguments);
+    if (false !== showDiff) showDiff = true;
+    if (undefined === expected && undefined === _actual) showDiff = false;
+    if (true !== config.showDiff) showDiff = false;
+
+    if (!ok) {
+      msg = util.getMessage(this, arguments);
+      var actual = util.getActual(this, arguments);
+      var assertionErrorObjectProperties = {
+        actual: actual,
+        expected: expected,
+        showDiff: showDiff,
+      };
+
+      var operator = util.getOperator(this, arguments);
+      if (operator) {
+        assertionErrorObjectProperties.operator = operator;
+      }
+
+      throw new AssertionError(
+        msg,
+        assertionErrorObjectProperties,
+        config.includeStack ? this.assert : util.flag(this, "ssfi"),
+      );
     }
+  }
 
-    throw new AssertionError(
-      msg,
-      assertionErrorObjectProperties,
-      config.includeStack ? this.assert : util.flag(this, 'ssfi')
-    );
+  /**
+   * Quick reference to stored `actual` value for plugin developers.
+   *
+   * @returns {unknown}
+   */
+  get _obj() {
+    return util.flag(this, "object");
   }
-};
 
-/**
- * ### ._obj
- *
- * Quick reference to stored `actual` value for plugin developers.
- *
- * @private
- */
-Object.defineProperty(Assertion.prototype, '_obj', {
-  get: function () {
-    return util.flag(this, 'object');
-  },
-  set: function (val) {
-    util.flag(this, 'object', val);
+  /**
+   * Quick reference to stored `actual` value for plugin developers.
+   *
+   * @param {unknown} val
+   */
+  set _obj(val) {
+    util.flag(this, "object", val);
   }
-});
+}
diff --git a/package-lock.json b/package-lock.json
index a3b43131..a6319ebf 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -26,7 +26,8 @@
         "eslint": "^8.56.0",
         "eslint-plugin-jsdoc": "^48.0.4",
         "mocha": "^10.2.0",
-        "prettier": "^3.4.2"
+        "prettier": "^3.4.2",
+        "typescript": "~5.7.3"
       },
       "engines": {
         "node": ">=12"
@@ -6700,6 +6701,20 @@
         "node": ">= 0.6"
       }
     },
+    "node_modules/typescript": {
+      "version": "5.7.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.7.3.tgz",
+      "integrity": "sha512-84MVSjMEHP+FQRPy3pX9sTVV/INIex71s9TL2Gm5FG/WG1SqXeKyZ0k7/blY/4FdOzI12CBy1vGc4og/eus0fw==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
     "node_modules/typical": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/typical/-/typical-4.0.0.tgz",
diff --git a/package.json b/package.json
index 79792a3d..cbf9ef25 100644
--- a/package.json
+++ b/package.json
@@ -39,6 +39,7 @@
     "lint": "npm run lint:js && npm run lint:format",
     "lint:js": "eslint lib/",
     "lint:format": "prettier --check lib",
+    "lint:types": "tsc",
     "clean": "rm -rf chai.js coverage/"
   },
   "engines": {
@@ -62,6 +63,7 @@
     "eslint": "^8.56.0",
     "eslint-plugin-jsdoc": "^48.0.4",
     "mocha": "^10.2.0",
-    "prettier": "^3.4.2"
+    "prettier": "^3.4.2",
+    "typescript": "~5.7.3"
   }
 }
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 00000000..3d9ba0e4
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,18 @@
+{
+  "compilerOptions": {
+    "target": "esnext",
+    "module": "nodenext",
+    "moduleResolution": "nodenext",
+    "types": [],
+    "checkJs": true,
+    "noEmit": true,
+    "isolatedModules": true,
+    "forceConsistentCasingInFileNames": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true
+  },
+  "include": [
+    "lib/**/*.js"
+  ]
+}