diff --git a/test/parallel/test-zlib-from-gzip.js b/test/parallel/test-zlib-from-gzip.js
index 7bd377eb8698fd..bca810e1a75d5f 100644
--- a/test/parallel/test-zlib-from-gzip.js
+++ b/test/parallel/test-zlib-from-gzip.js
@@ -21,10 +21,10 @@ const inp = fs.createReadStream(fixture);
 const out = fs.createWriteStream(outputFile);
 
 inp.pipe(gunzip).pipe(out);
-out.on('close', function() {
+out.on('close', common.mustCall(() => {
   const actual = fs.readFileSync(outputFile);
   assert.strictEqual(actual.length, expect.length, 'length should match');
   for (let i = 0, l = actual.length; i < l; i++) {
     assert.strictEqual(actual[i], expect[i], `byte[${i}]`);
   }
-});
+}));
diff --git a/test/parallel/test-zlib-from-string.js b/test/parallel/test-zlib-from-string.js
index ab2f7d023d8bc8..cf24d2a804cc73 100644
--- a/test/parallel/test-zlib-from-string.js
+++ b/test/parallel/test-zlib-from-string.js
@@ -1,7 +1,7 @@
 'use strict';
 // test compressing and uncompressing a string with zlib
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const zlib = require('zlib');
 
@@ -33,32 +33,32 @@ const expectedBase64Gzip = 'H4sIAAAAAAAAA11RS05DMQy8yhzg6d2BPSAkJPZu4laWkjiN4' +
                            'mHo33kJO8xfkckmLjE5XMKBQ4gxIsfvCZ44doUThF2mcZq8q2' +
                            'sHnHNzRtagj5AQAA';
 
-zlib.deflate(inputString, function(err, buffer) {
+zlib.deflate(inputString, common.mustCall((err, buffer) => {
   assert.strictEqual(buffer.toString('base64'), expectedBase64Deflate,
                      'deflate encoded string should match');
-});
+}));
 
-zlib.gzip(inputString, function(err, buffer) {
+zlib.gzip(inputString, common.mustCall((err, buffer) => {
   // Can't actually guarantee that we'll get exactly the same
   // deflated bytes when we compress a string, since the header
   // depends on stuff other than the input string itself.
   // However, decrypting it should definitely yield the same
   // result that we're expecting, and this should match what we get
   // from inflating the known valid deflate data.
-  zlib.gunzip(buffer, function(err, gunzipped) {
+  zlib.gunzip(buffer, common.mustCall((err, gunzipped) => {
     assert.strictEqual(gunzipped.toString(), inputString,
                        'Should get original string after gzip/gunzip');
-  });
-});
+  }));
+}));
 
 let buffer = Buffer.from(expectedBase64Deflate, 'base64');
-zlib.unzip(buffer, function(err, buffer) {
+zlib.unzip(buffer, common.mustCall((err, buffer) => {
   assert.strictEqual(buffer.toString(), inputString,
                      'decoded inflated string should match');
-});
+}));
 
 buffer = Buffer.from(expectedBase64Gzip, 'base64');
-zlib.unzip(buffer, function(err, buffer) {
+zlib.unzip(buffer, common.mustCall((err, buffer) => {
   assert.strictEqual(buffer.toString(), inputString,
                      'decoded gunzipped string should match');
-});
+}));
diff --git a/test/parallel/test-zlib-invalid-input.js b/test/parallel/test-zlib-invalid-input.js
index 6e87ab8b4f8227..d1d157678bba25 100644
--- a/test/parallel/test-zlib-invalid-input.js
+++ b/test/parallel/test-zlib-invalid-input.js
@@ -1,14 +1,26 @@
 'use strict';
 // test uncompressing invalid input
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const zlib = require('zlib');
 
-const nonStringInputs = [1, true, {a: 1}, ['a']];
+const nonStringInputs = [
+  1,
+  true,
+  { a: 1 },
+  ['a']
+];
 
-console.error('Doing the non-strings');
-nonStringInputs.forEach(function(input) {
+// zlib.Unzip classes need to get valid data, or else they'll throw.
+const unzips = [
+  zlib.Unzip(),
+  zlib.Gunzip(),
+  zlib.Inflate(),
+  zlib.InflateRaw()
+];
+
+nonStringInputs.forEach(common.mustCall((input) => {
   // zlib.gunzip should not throw an error when called with bad input.
   assert.doesNotThrow(function() {
     zlib.gunzip(input, function(err, buffer) {
@@ -16,30 +28,12 @@ nonStringInputs.forEach(function(input) {
       assert.ok(err);
     });
   });
-});
+}, nonStringInputs.length));
 
-console.error('Doing the unzips');
-// zlib.Unzip classes need to get valid data, or else they'll throw.
-const unzips = [ zlib.Unzip(),
-                 zlib.Gunzip(),
-                 zlib.Inflate(),
-                 zlib.InflateRaw() ];
-const hadError = [];
-unzips.forEach(function(uz, i) {
-  console.error(`Error for ${uz.constructor.name}`);
-  uz.on('error', function(er) {
-    console.error('Error event', er);
-    hadError[i] = true;
-  });
-
-  uz.on('end', function(er) {
-    throw new Error(`end event should not be emitted ${uz.constructor.name}`);
-  });
+unzips.forEach(common.mustCall((uz, i) => {
+  uz.on('error', common.mustCall());
+  uz.on('end', common.mustNotCall);
 
   // this will trigger error event
   uz.write('this is not valid compressed data.');
-});
-
-process.on('exit', function() {
-  assert.deepStrictEqual(hadError, [true, true, true, true], 'expect 4 errors');
-});
+}, unzips.length));
diff --git a/test/parallel/test-zlib-random-byte-pipes.js b/test/parallel/test-zlib-random-byte-pipes.js
index 143ab526f70a49..5ec5908777c814 100644
--- a/test/parallel/test-zlib-random-byte-pipes.js
+++ b/test/parallel/test-zlib-random-byte-pipes.js
@@ -6,124 +6,119 @@ if (!common.hasCrypto)
 const assert = require('assert');
 const crypto = require('crypto');
 const stream = require('stream');
-const util = require('util');
 const zlib = require('zlib');
 
 const Stream = stream.Stream;
 
 // emit random bytes, and keep a shasum
-function RandomReadStream(opt) {
-  Stream.call(this);
+class RandomReadStream extends Stream {
+  constructor(opt) {
+    super();
 
-  this.readable = true;
-  this._paused = false;
-  this._processing = false;
-
-  this._hasher = crypto.createHash('sha1');
-  opt = opt || {};
-
-  // base block size.
-  opt.block = opt.block || 256 * 1024;
+    this.readable = true;
+    this._paused = false;
+    this._processing = false;
 
-  // total number of bytes to emit
-  opt.total = opt.total || 256 * 1024 * 1024;
-  this._remaining = opt.total;
+    this._hasher = crypto.createHash('sha1');
+    opt = opt || {};
 
-  // how variable to make the block sizes
-  opt.jitter = opt.jitter || 1024;
+    // base block size.
+    opt.block = opt.block || 256 * 1024;
 
-  this._opt = opt;
+    // total number of bytes to emit
+    opt.total = opt.total || 256 * 1024 * 1024;
+    this._remaining = opt.total;
 
-  this._process = this._process.bind(this);
+    // how variable to make the block sizes
+    opt.jitter = opt.jitter || 1024;
 
-  process.nextTick(this._process);
-}
+    this._opt = opt;
 
-util.inherits(RandomReadStream, Stream);
+    this._process = this._process.bind(this);
 
-RandomReadStream.prototype.pause = function() {
-  this._paused = true;
-  this.emit('pause');
-};
+    process.nextTick(this._process);
+  }
 
-RandomReadStream.prototype.resume = function() {
-  // console.error("rrs resume");
-  this._paused = false;
-  this.emit('resume');
-  this._process();
-};
+  pause() {
+    this._paused = true;
+    this.emit('pause');
+  }
 
-RandomReadStream.prototype._process = function() {
-  if (this._processing) return;
-  if (this._paused) return;
+  resume() {
+    // console.error("rrs resume");
+    this._paused = false;
+    this.emit('resume');
+    this._process();
+  }
 
-  this._processing = true;
+  _process() {
+    if (this._processing) return;
+    if (this._paused) return;
 
-  if (!this._remaining) {
-    this._hash = this._hasher.digest('hex').toLowerCase().trim();
-    this._processing = false;
+    this._processing = true;
 
-    this.emit('end');
-    return;
-  }
+    if (!this._remaining) {
+      this._hash = this._hasher.digest('hex').toLowerCase().trim();
+      this._processing = false;
 
-  // figure out how many bytes to output
-  // if finished, then just emit end.
-  let block = this._opt.block;
-  const jitter = this._opt.jitter;
-  if (jitter) {
-    block += Math.ceil(Math.random() * jitter - (jitter / 2));
-  }
-  block = Math.min(block, this._remaining);
-  const buf = Buffer.allocUnsafe(block);
-  for (let i = 0; i < block; i++) {
-    buf[i] = Math.random() * 256;
-  }
+      this.emit('end');
+      return;
+    }
 
-  this._hasher.update(buf);
+    // figure out how many bytes to output
+    // if finished, then just emit end.
+    let block = this._opt.block;
+    const jitter = this._opt.jitter;
+    if (jitter) {
+      block += Math.ceil(Math.random() * jitter - (jitter / 2));
+    }
+    block = Math.min(block, this._remaining);
+    const buf = Buffer.allocUnsafe(block);
+    for (let i = 0; i < block; i++) {
+      buf[i] = Math.random() * 256;
+    }
 
-  this._remaining -= block;
+    this._hasher.update(buf);
 
-  console.error('block=%d\nremain=%d\n', block, this._remaining);
-  this._processing = false;
+    this._remaining -= block;
 
-  this.emit('data', buf);
-  process.nextTick(this._process);
-};
+    this._processing = false;
 
+    this.emit('data', buf);
+    process.nextTick(this._process);
+  }
+}
 
 // a filter that just verifies a shasum
-function HashStream() {
-  Stream.call(this);
+class HashStream extends Stream {
+  constructor() {
+    super();
+    this.readable = this.writable = true;
+    this._hasher = crypto.createHash('sha1');
+  }
 
-  this.readable = this.writable = true;
-  this._hasher = crypto.createHash('sha1');
-}
+  write(c) {
+    // Simulate the way that an fs.ReadStream returns false
+    // on *every* write, only to resume a moment later.
+    this._hasher.update(c);
+    process.nextTick(() => this.resume());
+    return false;
+  }
+
+  resume() {
+    this.emit('resume');
+    process.nextTick(() => this.emit('drain'));
+  }
 
-util.inherits(HashStream, Stream);
-
-HashStream.prototype.write = function(c) {
-  // Simulate the way that an fs.ReadStream returns false
-  // on *every* write like a jerk, only to resume a
-  // moment later.
-  this._hasher.update(c);
-  process.nextTick(this.resume.bind(this));
-  return false;
-};
-
-HashStream.prototype.resume = function() {
-  this.emit('resume');
-  process.nextTick(this.emit.bind(this, 'drain'));
-};
-
-HashStream.prototype.end = function(c) {
-  if (c) {
-    this.write(c);
+  end(c) {
+    if (c) {
+      this.write(c);
+    }
+    this._hash = this._hasher.digest('hex').toLowerCase().trim();
+    this.emit('data', this._hash);
+    this.emit('end');
   }
-  this._hash = this._hasher.digest('hex').toLowerCase().trim();
-  this.emit('data', this._hash);
-  this.emit('end');
-};
+}
 
 
 const inp = new RandomReadStream({ total: 1024, block: 256, jitter: 16 });
@@ -133,23 +128,6 @@ const gunz = zlib.createGunzip();
 
 inp.pipe(gzip).pipe(gunz).pipe(out);
 
-inp.on('data', function(c) {
-  console.error('inp data', c.length);
-});
-
-gzip.on('data', function(c) {
-  console.error('gzip data', c.length);
-});
-
-gunz.on('data', function(c) {
-  console.error('gunz data', c.length);
-});
-
-out.on('data', function(c) {
-  console.error('out data', c.length);
-});
-
-out.on('data', common.mustCall(function(c) {
-  console.error('hash=%s', c);
+out.on('data', common.mustCall((c) => {
   assert.strictEqual(c, inp._hash, 'hashes should match');
 }));
diff --git a/test/parallel/test-zlib-sync-no-event.js b/test/parallel/test-zlib-sync-no-event.js
index 33c0018b6f9108..9defd3d31f4fb0 100644
--- a/test/parallel/test-zlib-sync-no-event.js
+++ b/test/parallel/test-zlib-sync-no-event.js
@@ -1,20 +1,18 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const zlib = require('zlib');
 const assert = require('assert');
 
-const shouldNotBeCalled = () => { throw new Error('unexpected event'); };
-
 const message = 'Come on, Fhqwhgads.';
+const buffer = Buffer.from(message);
 
 const zipper = new zlib.Gzip();
-zipper.on('close', shouldNotBeCalled);
+zipper.on('close', common.mustNotCall);
 
-const buffer = Buffer.from(message);
 const zipped = zipper._processChunk(buffer, zlib.Z_FINISH);
 
 const unzipper = new zlib.Gunzip();
-unzipper.on('close', shouldNotBeCalled);
+unzipper.on('close', common.mustNotCall);
 
 const unzipped = unzipper._processChunk(zipped, zlib.Z_FINISH);
 assert.notStrictEqual(zipped.toString(), message);
diff --git a/test/parallel/test-zlib-write-after-flush.js b/test/parallel/test-zlib-write-after-flush.js
index caacc976e51a46..5ab601f7f1d46c 100644
--- a/test/parallel/test-zlib-write-after-flush.js
+++ b/test/parallel/test-zlib-write-after-flush.js
@@ -1,5 +1,5 @@
 'use strict';
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const zlib = require('zlib');
 
@@ -11,23 +11,14 @@ gzip.pipe(gunz);
 let output = '';
 const input = 'A line of data\n';
 gunz.setEncoding('utf8');
-gunz.on('data', function(c) {
-  output += c;
-});
-
-process.on('exit', function() {
+gunz.on('data', (c) => output += c);
+gunz.on('end', common.mustCall(() => {
   assert.strictEqual(output, input);
-
-  // Make sure that the flush flag was set back to normal
   assert.strictEqual(gzip._flushFlag, zlib.Z_NO_FLUSH);
-
-  console.log('ok');
-});
+}));
 
 // make sure that flush/write doesn't trigger an assert failure
-gzip.flush(); write();
-function write() {
-  gzip.write(input);
-  gzip.end();
-  gunz.read(0);
-}
+gzip.flush();
+gzip.write(input);
+gzip.end();
+gunz.read(0);
diff --git a/test/parallel/test-zlib.js b/test/parallel/test-zlib.js
index 468b5f346f04ee..dbd437e500b10f 100644
--- a/test/parallel/test-zlib.js
+++ b/test/parallel/test-zlib.js
@@ -4,7 +4,6 @@ const assert = require('assert');
 const zlib = require('zlib');
 const path = require('path');
 const fs = require('fs');
-const util = require('util');
 const stream = require('stream');
 
 let zlibPairs = [
@@ -48,105 +47,104 @@ if (process.env.FAST) {
 }
 
 const tests = {};
-testFiles.forEach(function(file) {
+testFiles.forEach(common.mustCall((file) => {
   tests[file] = fs.readFileSync(path.resolve(common.fixturesDir, file));
-});
+}, testFiles.length));
 
 
 // stream that saves everything
-function BufferStream() {
-  this.chunks = [];
-  this.length = 0;
-  this.writable = true;
-  this.readable = true;
+class BufferStream extends stream.Stream {
+  constructor() {
+    super();
+    this.chunks = [];
+    this.length = 0;
+    this.writable = true;
+    this.readable = true;
+  }
+
+  write(c) {
+    this.chunks.push(c);
+    this.length += c.length;
+    return true;
+  }
+
+  end(c) {
+    if (c) this.write(c);
+    // flatten
+    const buf = Buffer.allocUnsafe(this.length);
+    let i = 0;
+    this.chunks.forEach((c) => {
+      c.copy(buf, i);
+      i += c.length;
+    });
+    this.emit('data', buf);
+    this.emit('end');
+    return true;
+  }
 }
 
-util.inherits(BufferStream, stream.Stream);
-
-BufferStream.prototype.write = function(c) {
-  this.chunks.push(c);
-  this.length += c.length;
-  return true;
-};
-
-BufferStream.prototype.end = function(c) {
-  if (c) this.write(c);
-  // flatten
-  const buf = Buffer.allocUnsafe(this.length);
-  let i = 0;
-  this.chunks.forEach(function(c) {
-    c.copy(buf, i);
-    i += c.length;
-  });
-  this.emit('data', buf);
-  this.emit('end');
-  return true;
-};
-
-
-function SlowStream(trickle) {
-  this.trickle = trickle;
-  this.offset = 0;
-  this.readable = this.writable = true;
+class SlowStream extends stream.Stream {
+  constructor(trickle) {
+    super();
+    this.trickle = trickle;
+    this.offset = 0;
+    this.readable = this.writable = true;
+  }
+
+  write() {
+    throw new Error('not implemented, just call ss.end(chunk)');
+  }
+
+  pause() {
+    this.paused = true;
+    this.emit('pause');
+  }
+
+  resume() {
+    const emit = () => {
+      if (this.paused) return;
+      if (this.offset >= this.length) {
+        this.ended = true;
+        return this.emit('end');
+      }
+      const end = Math.min(this.offset + this.trickle, this.length);
+      const c = this.chunk.slice(this.offset, end);
+      this.offset += c.length;
+      this.emit('data', c);
+      process.nextTick(emit);
+    };
+
+    if (this.ended) return;
+    this.emit('resume');
+    if (!this.chunk) return;
+    this.paused = false;
+    emit();
+  }
+
+  end(chunk) {
+    // walk over the chunk in blocks.
+    this.chunk = chunk;
+    this.length = chunk.length;
+    this.resume();
+    return this.ended;
+  }
 }
 
-util.inherits(SlowStream, stream.Stream);
-
-SlowStream.prototype.write = function() {
-  throw new Error('not implemented, just call ss.end(chunk)');
-};
-
-SlowStream.prototype.pause = function() {
-  this.paused = true;
-  this.emit('pause');
-};
-
-SlowStream.prototype.resume = function() {
-  const emit = () => {
-    if (this.paused) return;
-    if (this.offset >= this.length) {
-      this.ended = true;
-      return this.emit('end');
-    }
-    const end = Math.min(this.offset + this.trickle, this.length);
-    const c = this.chunk.slice(this.offset, end);
-    this.offset += c.length;
-    this.emit('data', c);
-    process.nextTick(emit);
-  };
-
-  if (this.ended) return;
-  this.emit('resume');
-  if (!this.chunk) return;
-  this.paused = false;
-  emit();
-};
-
-SlowStream.prototype.end = function(chunk) {
-  // walk over the chunk in blocks.
-  this.chunk = chunk;
-  this.length = chunk.length;
-  this.resume();
-  return this.ended;
-};
-
 
 // for each of the files, make sure that compressing and
 // decompressing results in the same data, for every combination
 // of the options set above.
-let failures = 0;
-let total = 0;
-let done = 0;
 
-Object.keys(tests).forEach(function(file) {
+const testKeys = Object.keys(tests);
+testKeys.forEach(common.mustCall((file) => {
   const test = tests[file];
-  chunkSize.forEach(function(chunkSize) {
-    trickle.forEach(function(trickle) {
-      windowBits.forEach(function(windowBits) {
-        level.forEach(function(level) {
-          memLevel.forEach(function(memLevel) {
-            strategy.forEach(function(strategy) {
-              zlibPairs.forEach(function(pair) {
+  chunkSize.forEach(common.mustCall((chunkSize) => {
+    trickle.forEach(common.mustCall((trickle) => {
+      windowBits.forEach(common.mustCall((windowBits) => {
+        level.forEach(common.mustCall((level) => {
+          memLevel.forEach(common.mustCall((memLevel) => {
+            strategy.forEach(common.mustCall((strategy) => {
+              zlibPairs.forEach(common.mustCall((pair) => {
                 const Def = pair[0];
                 const Inf = pair[1];
                 const opts = { level: level,
@@ -154,57 +152,32 @@ Object.keys(tests).forEach(function(file) {
                                memLevel: memLevel,
                                strategy: strategy };
 
-                total++;
-
                 const def = new Def(opts);
                 const inf = new Inf(opts);
                 const ss = new SlowStream(trickle);
                 const buf = new BufferStream();
 
                 // verify that the same exact buffer comes out the other end.
-                buf.on('data', function(c) {
+                buf.on('data', common.mustCall((c) => {
                   const msg = `${file} ${chunkSize} ${
                     JSON.stringify(opts)} ${Def.name} -> ${Inf.name}`;
-                  let ok = true;
-                  const testNum = ++done;
                   let i;
                   for (i = 0; i < Math.max(c.length, test.length); i++) {
                     if (c[i] !== test[i]) {
-                      ok = false;
-                      failures++;
+                      assert.fail(null, null, msg);
                       break;
                     }
                   }
-                  if (ok) {
-                    console.log(`ok ${testNum} ${msg}`);
-                  } else {
-                    console.log(`not ok ${testNum} msg`);
-                    console.log('  ...');
-                    console.log(`  testfile: ${file}`);
-                    console.log(`  type: ${Def.name} -> ${Inf.name}`);
-                    console.log(`  position: ${i}`);
-                    console.log(`  options: ${JSON.stringify(opts)}`);
-                    console.log(`  expect: ${test[i]}`);
-                    console.log(`  actual: ${c[i]}`);
-                    console.log(`  chunkSize: ${chunkSize}`);
-                    console.log('  ---');
-                  }
-                });
+                }));
 
                 // the magic happens here.
                 ss.pipe(def).pipe(inf).pipe(buf);
                 ss.end(test);
-              });
-            });
-          });
-        });
-      });
-    });
-  });
-});
-
-process.on('exit', function(code) {
-  console.log(`1..${done}`);
-  assert.strictEqual(done, total, `${total - done} tests left unfinished`);
-  assert.strictEqual(failures, 0, 'some test failures');
-});
+              }, zlibPairs.length));
+            }, strategy.length));
+          }, memLevel.length));
+        }, level.length));
+      }, windowBits.length));
+    }, trickle.length));
+  }, chunkSize.length));
+}, testKeys.length));