diff --git a/test/parallel/test-timers-immediate.js b/test/parallel/test-timers-immediate.js
index f5e0fa00a725cc..0227e38efa9b20 100644
--- a/test/parallel/test-timers-immediate.js
+++ b/test/parallel/test-timers-immediate.js
@@ -23,9 +23,6 @@
 const common = require('../common');
 const assert = require('assert');
 
-let immediateC;
-let immediateD;
-
 let mainFinished = false;
 
 setImmediate(common.mustCall(function() {
@@ -35,17 +32,12 @@ setImmediate(common.mustCall(function() {
 
 const immediateB = setImmediate(common.mustNotCall());
 
-setImmediate(function(x, y, z) {
-  immediateC = [x, y, z];
-}, 1, 2, 3);
-
-setImmediate(function(x, y, z, a, b) {
-  immediateD = [x, y, z, a, b];
-}, 1, 2, 3, 4, 5);
+setImmediate(common.mustCall((...args) => {
+  assert.deepStrictEqual(args, [1, 2, 3]);
+}), 1, 2, 3);
 
-process.on('exit', function() {
-  assert.deepStrictEqual(immediateC, [1, 2, 3], 'immediateC args should match');
-  assert.deepStrictEqual(immediateD, [1, 2, 3, 4, 5], '5 args should match');
-});
+setImmediate(common.mustCall((...args) => {
+  assert.deepStrictEqual(args, [1, 2, 3, 4, 5]);
+}), 1, 2, 3, 4, 5);
 
 mainFinished = true;
diff --git a/test/parallel/test-timers-non-integer-delay.js b/test/parallel/test-timers-non-integer-delay.js
index bda4a31d81e495..017ef28d0f86ef 100644
--- a/test/parallel/test-timers-non-integer-delay.js
+++ b/test/parallel/test-timers-non-integer-delay.js
@@ -20,7 +20,7 @@
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 'use strict';
-require('../common');
+const common = require('../common');
 
 /*
  * This test makes sure that non-integer timer delays do not make the process
@@ -39,13 +39,11 @@ require('../common');
  */
 
 const TIMEOUT_DELAY = 1.1;
-const NB_TIMEOUTS_FIRED = 50;
+let N = 50;
 
-let nbTimeoutFired = 0;
-const interval = setInterval(function() {
-  ++nbTimeoutFired;
-  if (nbTimeoutFired === NB_TIMEOUTS_FIRED) {
+const interval = setInterval(common.mustCall(() => {
+  if (--N === 0) {
     clearInterval(interval);
     process.exit(0);
   }
-}, TIMEOUT_DELAY);
+}, N), TIMEOUT_DELAY);
diff --git a/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js b/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
index 5d57bbaae0011e..842c8180701aa1 100644
--- a/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
+++ b/test/parallel/test-timers-socket-timeout-removes-other-socket-unref-timer.js
@@ -6,6 +6,7 @@
 
 const common = require('../common');
 const net = require('net');
+const Countdown = require('../common/countdown');
 
 const clients = [];
 
@@ -19,7 +20,7 @@ const server = net.createServer(function onClient(client) {
      * the list of unref timers when traversing it, and exposes the
      * original issue in joyent/node#8897.
      */
-    clients[0].setTimeout(1, function onTimeout() {
+    clients[0].setTimeout(1, () => {
       clients[1].setTimeout(0);
       clients[0].end();
       clients[1].end();
@@ -31,19 +32,16 @@ const server = net.createServer(function onClient(client) {
   }
 });
 
-server.listen(0, common.localhostIPv4, function() {
-  let nbClientsEnded = 0;
+server.listen(0, common.localhostIPv4, common.mustCall(() => {
+  const countdown = new Countdown(2, common.mustCall(() => server.close()));
 
-  function addEndedClient(client) {
-    ++nbClientsEnded;
-    if (nbClientsEnded === 2) {
-      server.close();
-    }
+  {
+    const client = net.connect({ port: server.address().port });
+    client.on('end', () => countdown.dec());
   }
 
-  const client1 = net.connect({ port: this.address().port });
-  client1.on('end', addEndedClient);
-
-  const client2 = net.connect({ port: this.address().port });
-  client2.on('end', addEndedClient);
-});
+  {
+    const client = net.connect({ port: server.address().port });
+    client.on('end', () => countdown.dec());
+  }
+}));
diff --git a/test/parallel/test-timers-unref-leak.js b/test/parallel/test-timers-unref-leak.js
index 8eef00dd4ffe92..afecf7f15ce1b5 100644
--- a/test/parallel/test-timers-unref-leak.js
+++ b/test/parallel/test-timers-unref-leak.js
@@ -1,27 +1,14 @@
 'use strict';
-require('../common');
-const assert = require('assert');
+const common = require('../common');
 
-let called = 0;
-let closed = 0;
-
-const timeout = setTimeout(function() {
-  called++;
-}, 10);
+const timeout = setTimeout(common.mustCall(), 10);
 timeout.unref();
 
 // Wrap `close` method to check if the handle was closed
 const close = timeout._handle.close;
-timeout._handle.close = function() {
-  closed++;
+timeout._handle.close = common.mustCall(function() {
   return close.apply(this, arguments);
-};
+});
 
 // Just to keep process alive and let previous timer's handle die
-setTimeout(function() {
-}, 50);
-
-process.on('exit', function() {
-  assert.strictEqual(called, 1);
-  assert.strictEqual(closed, 1);
-});
+setTimeout(() => {}, 50);
diff --git a/test/parallel/test-timers-unref.js b/test/parallel/test-timers-unref.js
index 7f66a364e3f24b..0078d2dae352d5 100644
--- a/test/parallel/test-timers-unref.js
+++ b/test/parallel/test-timers-unref.js
@@ -21,60 +21,55 @@
 
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 
-let interval_fired = false;
-let timeout_fired = false;
 let unref_interval = false;
 let unref_timer = false;
-let unref_callbacks = 0;
 let checks = 0;
 
 const LONG_TIME = 10 * 1000;
 const SHORT_TIME = 100;
 
-assert.doesNotThrow(function() {
+assert.doesNotThrow(() => {
   setTimeout(() => {}, 10).unref().ref().unref();
 }, 'ref and unref are chainable');
 
-assert.doesNotThrow(function() {
+assert.doesNotThrow(() => {
   setInterval(() => {}, 10).unref().ref().unref();
 }, 'ref and unref are chainable');
 
-setInterval(function() {
-  interval_fired = true;
-}, LONG_TIME).unref();
+setInterval(common.mustNotCall('Interval should not fire'), LONG_TIME).unref();
+setTimeout(common.mustNotCall('Timer should not fire'), LONG_TIME).unref();
 
-setTimeout(function() {
-  timeout_fired = true;
-}, LONG_TIME).unref();
-
-const interval = setInterval(function() {
+const interval = setInterval(common.mustCall(() => {
   unref_interval = true;
   clearInterval(interval);
-}, SHORT_TIME);
+}), SHORT_TIME);
 interval.unref();
 
-setTimeout(function() {
+setTimeout(common.mustCall(() => {
   unref_timer = true;
-}, SHORT_TIME).unref();
+}), SHORT_TIME).unref();
 
-const check_unref = setInterval(function() {
+const check_unref = setInterval(() => {
   if (checks > 5 || (unref_interval && unref_timer))
     clearInterval(check_unref);
   checks += 1;
 }, 100);
 
-setTimeout(function() {
-  unref_callbacks++;
-  this.unref();
-}, SHORT_TIME);
+{
+  const timeout =
+    setTimeout(common.mustCall(() => {
+      timeout.unref();
+    }), SHORT_TIME);
+}
 
-// Should not timeout the test
-setInterval(function() {
-  this.unref();
-}, SHORT_TIME);
+{
+  // Should not timeout the test
+  const timeout =
+    setInterval(() => timeout.unref(), SHORT_TIME);
+}
 
 // Should not assert on args.Holder()->InternalFieldCount() > 0. See #4261.
 {
@@ -82,16 +77,3 @@ setInterval(function() {
   process.nextTick(t.unref.bind({}));
   process.nextTick(t.unref.bind(t));
 }
-
-process.on('exit', function() {
-  assert.strictEqual(interval_fired, false,
-                     'Interval should not fire');
-  assert.strictEqual(timeout_fired, false,
-                     'Timeout should not fire');
-  assert.strictEqual(unref_timer, true,
-                     'An unrefd timeout should still fire');
-  assert.strictEqual(unref_interval, true,
-                     'An unrefd interval should still fire');
-  assert.strictEqual(unref_callbacks, 1,
-                     'Callback should only run once');
-});
diff --git a/test/parallel/test-timers-unrefd-interval-still-fires.js b/test/parallel/test-timers-unrefd-interval-still-fires.js
index bf16013f004965..a9e9af84304217 100644
--- a/test/parallel/test-timers-unrefd-interval-still-fires.js
+++ b/test/parallel/test-timers-unrefd-interval-still-fires.js
@@ -5,23 +5,20 @@
 const common = require('../common');
 
 const TEST_DURATION = common.platformTimeout(1000);
-const N = 3;
-let nbIntervalFired = 0;
+let N = 3;
 
-const keepOpen = setTimeout(() => {
-  console.error('[FAIL] Interval fired %d/%d times.', nbIntervalFired, N);
-  throw new Error('Test timed out. keepOpen was not canceled.');
-}, TEST_DURATION);
+const keepOpen =
+  setTimeout(
+    common.mustNotCall('Test timed out. keepOpen was not canceled.'),
+    TEST_DURATION);
 
-const timer = setInterval(() => {
-  ++nbIntervalFired;
-  if (nbIntervalFired === N) {
+const timer = setInterval(common.mustCall(() => {
+  if (--N === 0) {
     clearInterval(timer);
-    timer._onTimeout = () => {
-      throw new Error('Unrefd interval fired after being cleared.');
-    };
+    timer._onTimeout =
+      common.mustNotCall('Unrefd interal fired after being cleared');
     clearTimeout(keepOpen);
   }
-}, 1);
+}, N), 1);
 
 timer.unref();
diff --git a/test/parallel/test-timers-unrefed-in-beforeexit.js b/test/parallel/test-timers-unrefed-in-beforeexit.js
index 530d97674d8bf7..a38b55bf45d599 100644
--- a/test/parallel/test-timers-unrefed-in-beforeexit.js
+++ b/test/parallel/test-timers-unrefed-in-beforeexit.js
@@ -1,20 +1,7 @@
 'use strict';
 
-require('../common');
-const assert = require('assert');
+const common = require('../common');
 
-let once = 0;
-
-process.on('beforeExit', () => {
-  if (once > 1)
-    throw new RangeError('beforeExit should only have been called once!');
-
-  setTimeout(() => {}, 1).unref();
-  once++;
-});
-
-process.on('exit', (code) => {
-  if (code !== 0) return;
-
-  assert.strictEqual(once, 1);
-});
+process.on('beforeExit', common.mustCall(() => {
+  setTimeout(common.mustNotCall(), 1).unref();
+}));
diff --git a/test/parallel/test-timers-zero-timeout.js b/test/parallel/test-timers-zero-timeout.js
index 7feb01854e0d64..61a5b2131bad57 100644
--- a/test/parallel/test-timers-zero-timeout.js
+++ b/test/parallel/test-timers-zero-timeout.js
@@ -36,18 +36,14 @@ const assert = require('assert');
 }
 
 {
-  let ncalled = 0;
+  let ncalled = 3;
 
-  const iv = setInterval(f, 0, 'foo', 'bar', 'baz');
-
-  function f(a, b, c) {
+  const f = common.mustCall((a, b, c) => {
     assert.strictEqual(a, 'foo');
     assert.strictEqual(b, 'bar');
     assert.strictEqual(c, 'baz');
-    if (++ncalled === 3) clearTimeout(iv);
-  }
+    if (--ncalled === 0) clearTimeout(iv);
+  }, ncalled);
 
-  process.on('exit', function() {
-    assert.strictEqual(ncalled, 3);
-  });
+  const iv = setInterval(f, 0, 'foo', 'bar', 'baz');
 }