diff --git a/.eslintrc.yml b/.eslintrc.yml
index 34c77cabe4..43d8bcfd9e 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -14,6 +14,11 @@ rules:
   strict:
     - error
     - safe
+  # disallow Object.assign
+  no-restricted-properties:
+    - error
+    - object: 'Object'
+      property: 'assign'
 overrides:
   - files:
       - scripts/**/*.js
@@ -31,6 +36,11 @@ overrides:
       ecmaVersion: 2018
     env:
       browser: false
+    rules:
+      no-restricted-properties:
+        - off
+        - object: 'Object'
+          property: 'assign'
   - files:
       - esm-utils.js
     parserOptions:
@@ -79,7 +89,6 @@ overrides:
         # disallow property access of `global.<timer>.*`
         - selector: '*[object.object.name=global][object.property.name=/(Date|(set|clear)(Timeout|Immediate|Interval))/]:expression'
           message: *GH-237
-
   - files:
       - test/**/*.mjs
     parserOptions:
diff --git a/test/unit/mocha.spec.js b/test/unit/mocha.spec.js
index 4442af73f8..96aad6e66c 100644
--- a/test/unit/mocha.spec.js
+++ b/test/unit/mocha.spec.js
@@ -52,7 +52,7 @@ describe('Mocha', function() {
     reporterInstance = {};
     opts = {reporter: sandbox.stub().returns(reporterInstance)};
     Base = sandbox.stub().returns({});
-    runner = Object.assign(sandbox.createStubInstance(EventEmitter), {
+    runner = utils.assign(sandbox.createStubInstance(EventEmitter), {
       run: sandbox
         .stub()
         .callsArgAsync(0)
@@ -65,7 +65,7 @@ describe('Mocha', function() {
     // the Runner constructor is the main export, and constants is a static prop.
     // we don't need the constants themselves, but the object cannot be undefined
     Runner.constants = {};
-    suite = Object.assign(sandbox.createStubInstance(EventEmitter), {
+    suite = utils.assign(sandbox.createStubInstance(EventEmitter), {
       slow: sandbox.stub(),
       timeout: sandbox.stub(),
       bail: sandbox.stub(),
@@ -680,8 +680,6 @@ describe('Mocha', function() {
       });
 
       describe('when the `Mocha` instance is already disposed', function() {
-        var mocha;
-
         beforeEach(function() {
           mocha.dispose();
         });
@@ -713,8 +711,6 @@ describe('Mocha', function() {
       });
 
       describe('when a run has finished and is called again', function() {
-        var mocha;
-
         beforeEach(function(done) {
           mocha.run(function() {
             runner.run.reset();
@@ -748,8 +744,6 @@ describe('Mocha', function() {
       });
 
       describe('when Mocha configured for multiple runs and multiple runs are attempted', function() {
-        var mocha;
-
         beforeEach(function() {
           mocha.cleanReferencesAfterRun(false);
         });