From 11c45609b56dda11460b1f8e0d2a415cf8f9915d Mon Sep 17 00:00:00 2001
From: Mattias Norlander <mattias.norlander@gmail.com>
Date: Tue, 15 Feb 2022 15:02:15 +0100
Subject: [PATCH] fix: configurable max diff size (#4799)

---
 docs/index.md               | 318 +++++++++++++++++++-----------------
 lib/reporters/base.js       |  30 +++-
 test/reporters/base.spec.js |  73 +++++++--
 3 files changed, 250 insertions(+), 171 deletions(-)

diff --git a/docs/index.md b/docs/index.md
index be27aa9a3b..28ce96ffde 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -78,9 +78,9 @@ In your editor:
 
 ```js
 var assert = require('assert');
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it('should return -1 when the value is not present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it('should return -1 when the value is not present', function () {
       assert.equal([1, 2, 3].indexOf(4), -1);
     });
   });
@@ -174,7 +174,7 @@ In a browser, test files are loaded by `<script>` tags, and calling `mocha.run()
 If you use callback-based async tests, Mocha will throw an error if `done()` is called multiple times. This is handy for catching accidental double callbacks.
 
 ```javascript
-it('double done', function(done) {
+it('double done', function (done) {
   // Calling `done()` twice is an error
   setImmediate(done);
   setImmediate(done);
@@ -217,11 +217,11 @@ Mocha allows you to use any assertion library you wish. In the above example, we
 By adding an argument (usually named `done`) to `it()` to a test callback, Mocha will know that it should wait for this function to be called to complete the test. This callback accepts both an `Error` instance (or subclass thereof) _or_ a falsy value; anything else is invalid usage and throws an error (usually causing a failed test).
 
 ```js
-describe('User', function() {
-  describe('#save()', function() {
-    it('should save without error', function(done) {
+describe('User', function () {
+  describe('#save()', function () {
+    it('should save without error', function (done) {
       var user = new User('Luna');
-      user.save(function(err) {
+      user.save(function (err) {
         if (err) done(err);
         else done();
       });
@@ -233,9 +233,9 @@ describe('User', function() {
 Alternatively, use the `done()` callback directly (which will handle an error argument, if it exists):
 
 ```js
-describe('User', function() {
-  describe('#save()', function() {
-    it('should save without error', function(done) {
+describe('User', function () {
+  describe('#save()', function () {
+    it('should save without error', function (done) {
       var user = new User('Luna');
       user.save(done);
     });
@@ -248,14 +248,14 @@ describe('User', function() {
 Alternately, instead of using the `done()` callback, you may return a [Promise][mdn-promise]. This is useful if the APIs you are testing return promises instead of taking callbacks:
 
 ```js
-beforeEach(function() {
-  return db.clear().then(function() {
+beforeEach(function () {
+  return db.clear().then(function () {
     return db.save([tobi, loki, jane]);
   });
 });
 
-describe('#find()', function() {
-  it('respond with matching records', function() {
+describe('#find()', function () {
+  it('respond with matching records', function () {
     return db.find({type: 'User'}).should.eventually.have.length(3);
   });
 });
@@ -269,8 +269,8 @@ In Mocha v3.0.0 and newer, returning a `Promise` _and_ calling `done()` will res
 const assert = require('assert');
 
 // antipattern
-it('should complete this test', function(done) {
-  return new Promise(function(resolve) {
+it('should complete this test', function (done) {
+  return new Promise(function (resolve) {
     assert.ok(true);
     resolve();
   }).then(done);
@@ -284,13 +284,13 @@ The above test will fail with `Error: Resolution method is overspecified. Specif
 If your JS environment supports [async / await][mdn-async], you can also write asynchronous tests like this:
 
 ```js
-beforeEach(async function() {
+beforeEach(async function () {
   await db.clear();
   await db.save([tobi, loki, jane]);
 });
 
-describe('#find()', function() {
-  it('responds with matching records', async function() {
+describe('#find()', function () {
+  it('responds with matching records', async function () {
     const users = await db.find({type: 'User'});
     users.should.have.length(3);
   });
@@ -302,9 +302,9 @@ describe('#find()', function() {
 When testing synchronous code, omit the callback and Mocha will automatically continue on to the next test.
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it('should return -1 when the value is not present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it('should return -1 when the value is not present', function () {
       [1, 2, 3].indexOf(5).should.equal(-1);
       [1, 2, 3].indexOf(0).should.equal(-1);
     });
@@ -333,20 +333,20 @@ _If you do not need to use_ Mocha's context, lambdas should work. Be aware that
 With its default "BDD"-style interface, Mocha provides the hooks `before()`, `after()`, `beforeEach()`, and `afterEach()`. These should be used to set up preconditions and clean up after your tests.
 
 ```js
-describe('hooks', function() {
-  before(function() {
+describe('hooks', function () {
+  before(function () {
     // runs once before the first test in this block
   });
 
-  after(function() {
+  after(function () {
     // runs once after the last test in this block
   });
 
-  beforeEach(function() {
+  beforeEach(function () {
     // runs before each test in this block
   });
 
-  afterEach(function() {
+  afterEach(function () {
     // runs after each test in this block
   });
 
@@ -361,7 +361,7 @@ describe('hooks', function() {
 Any hook can be invoked with an optional description, making it easier to pinpoint errors in your tests. If a hook is given a named function, that name will be used if no description is supplied.
 
 ```js
-beforeEach(function() {
+beforeEach(function () {
   // beforeEach hook
 });
 
@@ -369,7 +369,7 @@ beforeEach(function namedFun() {
   // beforeEach:namedFun
 });
 
-beforeEach('some description', function() {
+beforeEach('some description', function () {
   // beforeEach:some description
 });
 ```
@@ -379,22 +379,22 @@ beforeEach('some description', function() {
 All hooks (`before()`, `after()`, `beforeEach()`, `afterEach()`) may be sync or async as well, behaving much like a regular test-case. For example, you may wish to populate database with dummy content before each test:
 
 ```js
-describe('Connection', function() {
+describe('Connection', function () {
   var db = new Connection(),
     tobi = new User('tobi'),
     loki = new User('loki'),
     jane = new User('jane');
 
-  beforeEach(function(done) {
-    db.clear(function(err) {
+  beforeEach(function (done) {
+    db.clear(function (err) {
       if (err) return done(err);
       db.save([tobi, loki, jane], done);
     });
   });
 
-  describe('#find()', function() {
-    it('respond with matching records', function(done) {
-      db.find({type: 'User'}, function(err, res) {
+  describe('#find()', function () {
+    it('respond with matching records', function (done) {
+      db.find({type: 'User'}, function (err, res) {
         if (err) return done(err);
         res.should.have.length(3);
         done();
@@ -419,18 +419,20 @@ If you need to perform asynchronous operations before any of your suites are run
 ```js
 const assert = require('assert');
 
-const fn = async x => { return new Promise(
-  resolve => { setTimeout(resolve, 3000, 2 * x); }
-)};
+const fn = async x => {
+  return new Promise(resolve => {
+    setTimeout(resolve, 3000, 2 * x);
+  });
+};
 
 // instead of an IIFE, you can use 'setImmediate' or 'nextTick' or 'setTimeout'
-(async function() {
+(async function () {
   const z = await fn(3);
 
-  describe('my suite', function() {
-      it(`expected value ${z}`, function() {
-          assert.strictEqual(z, 6);
-      });
+  describe('my suite', function () {
+    it(`expected value ${z}`, function () {
+      assert.strictEqual(z, 6);
+    });
   });
 
   run();
@@ -442,8 +444,8 @@ const fn = async x => { return new Promise(
 "Pending" — as in "someone should write these test cases eventually" — test-cases are those _without_ a callback:
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
     // pending test below
     it('should return -1 when the value is not present');
   });
@@ -462,8 +464,8 @@ The exclusivity feature allows you to run _only_ the specified suite or test-cas
 by appending `.only()` to the function. Here's an example of executing only a particular suite:
 
 ```js
-describe('Array', function() {
-  describe.only('#indexOf()', function() {
+describe('Array', function () {
+  describe.only('#indexOf()', function () {
     // ...
   });
 });
@@ -474,13 +476,13 @@ _Note_: All nested suites will still be executed.
 Here's an example of executing an individual test case:
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it.only('should return -1 unless present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it.only('should return -1 unless present', function () {
       // ...
     });
 
-    it('should return the index when present', function() {
+    it('should return the index when present', function () {
       // ...
     });
   });
@@ -490,17 +492,17 @@ describe('Array', function() {
 Previous to v3.0.0, `.only()` used string matching to decide which tests to execute; this is no longer the case. In v3.0.0 or newer, `.only()` can be used multiple times to define a subset of tests to run:
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it.only('should return -1 unless present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it.only('should return -1 unless present', function () {
       // this test will be run
     });
 
-    it.only('should return the index when present', function() {
+    it.only('should return the index when present', function () {
       // this test will also be run
     });
 
-    it('should return -1 if called with a non-Array context', function() {
+    it('should return -1 if called with a non-Array context', function () {
       // this test will not be run
     });
   });
@@ -510,25 +512,25 @@ describe('Array', function() {
 You may also choose multiple suites:
 
 ```js
-describe('Array', function() {
-  describe.only('#indexOf()', function() {
-    it('should return -1 unless present', function() {
+describe('Array', function () {
+  describe.only('#indexOf()', function () {
+    it('should return -1 unless present', function () {
       // this test will be run
     });
 
-    it('should return the index when present', function() {
+    it('should return the index when present', function () {
       // this test will also be run
     });
   });
 
-  describe.only('#concat()', function() {
-    it('should return a new Array', function() {
+  describe.only('#concat()', function () {
+    it('should return a new Array', function () {
       // this test will also be run
     });
   });
 
-  describe('#slice()', function() {
-    it('should return a new Array', function() {
+  describe('#slice()', function () {
+    it('should return a new Array', function () {
       // this test will not be run
     });
   });
@@ -538,13 +540,13 @@ describe('Array', function() {
 But _tests will have precedence_:
 
 ```js
-describe('Array', function() {
-  describe.only('#indexOf()', function() {
-    it.only('should return -1 unless present', function() {
+describe('Array', function () {
+  describe.only('#indexOf()', function () {
+    it.only('should return -1 unless present', function () {
       // this test will be run
     });
 
-    it('should return the index when present', function() {
+    it('should return the index when present', function () {
       // this test will not be run
     });
   });
@@ -560,13 +562,13 @@ _Note_: Hooks, if present, will still be executed.
 This feature is the inverse of `.only()`. By appending `.skip()`, you may tell Mocha to ignore test case(s). Anything skipped will be marked as [pending](#pending-tests), and reported as such. Here's an example of skipping an individual test:
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it.skip('should return -1 unless present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it.skip('should return -1 unless present', function () {
       // this test will not be run
     });
 
-    it('should return the index when present', function() {
+    it('should return the index when present', function () {
       // this test will be run
     });
   });
@@ -576,9 +578,9 @@ describe('Array', function() {
 You can also put `.skip()` on an entire suite. This is equivalent to appending `.skip()` onto all tests in the suite. Hooks in the suite are also skipped.
 
 ```js
-describe('Array', function() {
-  describe.skip('#indexOf()', function() {
-    it('should return -1 unless present', function() {
+describe('Array', function () {
+  describe.skip('#indexOf()', function () {
+    it('should return -1 unless present', function () {
       // this test will not be run
     });
   });
@@ -636,21 +638,21 @@ before(function() {
 This will skip all `it`, `beforeEach/afterEach`, and `describe` blocks within the suite. `before/after` hooks are skipped unless they are defined at the same level as the hook containing `this.skip()`.
 
 ```js
-describe('outer', function() {
-  before(function() {
+describe('outer', function () {
+  before(function () {
     this.skip();
   });
 
-  after(function() {
+  after(function () {
     // will be executed
   });
 
-  describe('inner', function() {
-    before(function() {
+  describe('inner', function () {
+    before(function () {
       // will be skipped
     });
 
-    after(function() {
+    after(function () {
       // will be skipped
     });
   });
@@ -670,15 +672,15 @@ This feature does re-run a failed test and its corresponding `beforeEach/afterEa
 **NOTE**: Example below was written using Selenium webdriver (which [overwrites global Mocha hooks][selenium-webdriver-testing] for `Promise` chain).
 
 ```js
-describe('retries', function() {
+describe('retries', function () {
   // Retry all tests in this suite up to 4 times
   this.retries(4);
 
-  beforeEach(function() {
+  beforeEach(function () {
     browser.get('http://www.yahoo.com');
   });
 
-  it('should succeed on the 3rd try', function() {
+  it('should succeed on the 3rd try', function () {
     // Specify this test to only retry up to 2 times
     this.retries(2);
     expect($('.foo').isDisplayed()).to.eventually.be.true;
@@ -699,7 +701,7 @@ function add(args) {
   return args.reduce((prev, curr) => prev + curr, 0);
 }
 
-describe('add()', function() {
+describe('add()', function () {
   const tests = [
     {args: [1, 2], expected: 3},
     {args: [1, 2, 3], expected: 6},
@@ -707,7 +709,7 @@ describe('add()', function() {
   ];
 
   tests.forEach(({args, expected}) => {
-    it(`correctly adds ${args.length} args`, function() {
+    it(`correctly adds ${args.length} args`, function () {
       const res = add(args);
       assert.strictEqual(res, expected);
     });
@@ -730,9 +732,9 @@ Tests added inside a `.forEach` handler often don't play well with editor plugin
 Another way to parameterize tests is to generate them with a closure. This following example is equivalent to the one above:
 
 ```js
-describe('add()', function() {
+describe('add()', function () {
   const testAdd = ({args, expected}) =>
-    function() {
+    function () {
       const res = add(args);
       assert.strictEqual(res, expected);
     };
@@ -753,16 +755,16 @@ import assert from 'assert';
 // top-level await: Node >= v14.8.0 with ESM test file
 const tests = await new Promise(resolve => {
   setTimeout(resolve, 5000, [
-      {args: [1, 2], expected: 3},
-      {args: [1, 2, 3], expected: 6},
-      {args: [1, 2, 3, 4], expected: 10}
+    {args: [1, 2], expected: 3},
+    {args: [1, 2, 3], expected: 6},
+    {args: [1, 2, 3, 4], expected: 10}
   ]);
 });
 
 // in suites ASYNCHRONOUS callbacks are NOT supported
-describe('add()', function() {
+describe('add()', function () {
   tests.forEach(({args, expected}) => {
-    it(`correctly adds ${args.length} args`, function() {
+    it(`correctly adds ${args.length} args`, function () {
       const res = args.reduce((sum, curr) => sum + curr, 0);
       assert.strictEqual(res, expected);
     });
@@ -787,10 +789,10 @@ There are three levels of test duration (depicted in the following image):
 To tweak what's considered "slow", you can use the `slow()` method:
 
 ```js
-describe('something slow', function() {
+describe('something slow', function () {
   this.slow(300000); // five minutes
 
-  it('should take long enough for me to go make a sandwich', function() {
+  it('should take long enough for me to go make a sandwich', function () {
     // ...
   });
 });
@@ -803,14 +805,14 @@ describe('something slow', function() {
 Suite-level timeouts may be applied to entire test "suites", or disabled via `this.timeout(0)`. This will be inherited by all nested suites and test-cases that do not override the value.
 
 ```js
-describe('a suite of tests', function() {
+describe('a suite of tests', function () {
   this.timeout(500);
 
-  it('should take less than 500ms', function(done) {
+  it('should take less than 500ms', function (done) {
     setTimeout(done, 300);
   });
 
-  it('should take less than 500ms as well', function(done) {
+  it('should take less than 500ms as well', function (done) {
     setTimeout(done, 250);
   });
 });
@@ -821,7 +823,7 @@ describe('a suite of tests', function() {
 Test-specific timeouts may also be applied, or the use of `this.timeout(0)` to disable timeouts all together:
 
 ```js
-it('should take less than 500ms', function(done) {
+it('should take less than 500ms', function (done) {
   this.timeout(500);
   setTimeout(done, 300);
 });
@@ -832,8 +834,8 @@ it('should take less than 500ms', function(done) {
 Hook-level timeouts may also be applied:
 
 ```js
-describe('a suite of tests', function() {
-  beforeEach(function(done) {
+describe('a suite of tests', function () {
+  beforeEach(function (done) {
     this.timeout(3000); // A very long environment setup.
     setTimeout(done, 2500);
   });
@@ -981,6 +983,16 @@ Some assertion libraries will supply their own diffs, in which case Mocha's will
 
 Mocha's own diff output does not conform to any known standards, and is designed to be human-readable.
 
+> _New in v9.2.1_
+
+By default strings are truncated to 8192 characters before generating a diff. This is to prevent performance problems with large strings.
+
+It can however make the output harder to interpret when comparing large strings. Therefore it is possible to configure this value using `--reporter-option maxDiffSize=[number]`.
+
+A value of 0 indicates no limit, default is 8192 characters.
+
+## Command-Line Usage
+
 ### `--full-trace`
 
 Enable "full" stack traces. By default, Mocha attempts to distill stack traces into less noisy (though still useful) output.
@@ -1154,17 +1166,17 @@ And another option with double quotes: `--grep "groupA|groupB"`.<br>
 And for more complex criterias: `--grep "/get/i"`. Some shells as e.g. Git-Bash-for-Windows may require: `--grep "'/get/i'"`. Using flags other than the `ignoreCase /i` (especially `/g` and `/y`) may lead to unpredictable results.
 
 ```js
-describe('api', function() {
-  describe('GET /api/users groupA', function() {
-    it('respond with an array of users', function() {
+describe('api', function () {
+  describe('GET /api/users groupA', function () {
+    it('respond with an array of users', function () {
       // ...
     });
   });
 });
 
-describe('app', function() {
-  describe('GET /users groupB', function() {
-    it('respond with an array of users', function() {
+describe('app', function () {
+  describe('GET /users groupB', function () {
+    it('respond with an array of users', function () {
       // ...
     });
   });
@@ -1319,12 +1331,12 @@ A _root hook_ is a hook in a test file which is _not defined_ within a suite. An
 // test/setup.js
 
 // root hook to run before every test (even in other files)
-beforeEach(function() {
+beforeEach(function () {
   doMySetup();
 });
 
 // root hook to run after every test (even in other files)
-afterEach(function() {
+afterEach(function () {
   doMyTeardown();
 });
 ```
@@ -1478,11 +1490,11 @@ Multiple root hooks can be defined in a single plugin, for organizational purpos
 
 export const mochaHooks = {
   beforeEach: [
-    function(done) {
+    function (done) {
       // do something before every test,
       // then run the next hook in this array
     },
-    async function() {
+    async function () {
       // async or Promise-returning functions allowed
     }
   ]
@@ -1501,10 +1513,10 @@ export const mochaHooks = () => {
     // root hooks object
     return {
       beforeEach: [
-        function() {
+        function () {
           // CI-specific beforeEach
         },
-        function() {
+        function () {
           // some other CI-specific beforeEach
         }
       ]
@@ -1557,16 +1569,16 @@ For example, given the following file, `test/test.spec.js`, containing root hook
 ```js
 // test/test.spec.js
 
-beforeEach(function() {
+beforeEach(function () {
   // global setup for all tests
 });
 
-after(function() {
+after(function () {
   // one-time final cleanup
 });
 
-describe('my test suite', function() {
-  it('should have run my global setup', function() {
+describe('my test suite', function () {
+  it('should have run my global setup', function () {
     // make assertion
   });
 });
@@ -1578,10 +1590,10 @@ Your `test/hooks.js` (for this example, a CJS module) should contain:
 // test/hooks.js
 
 exports.mochaHooks = {
-  beforeEach: function() {
+  beforeEach: function () {
     // global setup for all tests
   },
-  afterAll: function() {
+  afterAll: function () {
     // one-time final cleanup
   }
 };
@@ -1594,8 +1606,8 @@ Your original `test/test.spec.js` should now contain:
 ```js
 // test/test.spec.js
 
-describe('my test suite', function() {
-  it('should have run my global setup', function() {
+describe('my test suite', function () {
+  it('should have run my global setup', function () {
     // make assertion
   });
 });
@@ -1631,7 +1643,7 @@ To create a global setup fixture, export `mochaGlobalSetup` from a script, e.g.,
 // fixtures.cjs
 
 // can be async or not
-exports.mochaGlobalSetup = async function() {
+exports.mochaGlobalSetup = async function () {
   this.server = await startSomeServer({port: process.env.TEST_PORT});
   console.log(`server running on port ${this.server.port}`);
 };
@@ -1663,7 +1675,7 @@ Just like a [global setup fixture](#global-setup-fixtures), a _global teardown f
 // fixtures.cjs, cont'd
 
 // can be async or not
-exports.mochaGlobalTeardown = async function() {
+exports.mochaGlobalTeardown = async function () {
   await this.server.stop();
   console.log('server stopped!');
 };
@@ -1724,18 +1736,18 @@ Then, connect to the server in your tests:
 
 import {connect} from 'my-server-connector-thingy';
 
-describe('my API', function() {
+describe('my API', function () {
   let connection;
 
-  before(async function() {
+  before(async function () {
     connection = await connect({port: process.env.TEST_PORT});
   });
 
-  it('should be a nice API', function() {
+  it('should be a nice API', function () {
     // assertions here
   });
 
-  after(async function() {
+  after(async function () {
     return connection.close();
   });
 });
@@ -1763,24 +1775,24 @@ The **BDD** interface provides `describe()`, `context()`, `it()`, `specify()`, `
 > All of the previous examples were written using the **BDD** interface.
 
 ```js
-describe('Array', function() {
-  before(function() {
+describe('Array', function () {
+  before(function () {
     // ...
   });
 
-  describe('#indexOf()', function() {
-    context('when not present', function() {
-      it('should not throw an error', function() {
-        (function() {
+  describe('#indexOf()', function () {
+    context('when not present', function () {
+      it('should not throw an error', function () {
+        (function () {
           [1, 2, 3].indexOf(4);
         }.should.not.throw());
       });
-      it('should return -1', function() {
+      it('should return -1', function () {
         [1, 2, 3].indexOf(4).should.equal(-1);
       });
     });
-    context('when present', function() {
-      it('should return the index where the element first appears in the array', function() {
+    context('when present', function () {
+      it('should return the index where the element first appears in the array', function () {
         [1, 2, 3].indexOf(3).should.equal(2);
       });
     });
@@ -1793,13 +1805,13 @@ describe('Array', function() {
 The **TDD** interface provides `suite()`, `test()`, `suiteSetup()`, `suiteTeardown()`, `setup()`, and `teardown()`:
 
 ```js
-suite('Array', function() {
-  setup(function() {
+suite('Array', function () {
+  setup(function () {
     // ...
   });
 
-  suite('#indexOf()', function() {
-    test('should return -1 when not present', function() {
+  suite('#indexOf()', function () {
+    test('should return -1 when not present', function () {
       assert.equal(-1, [1, 2, 3].indexOf(4));
     });
   });
@@ -1812,13 +1824,13 @@ The **Exports** interface is much like Mocha's predecessor [expresso][]. The key
 
 ```js
 module.exports = {
-  before: function() {
+  before: function () {
     // ...
   },
 
   Array: {
     '#indexOf()': {
-      'should return -1 when not present': function() {
+      'should return -1 when not present': function () {
         [1, 2, 3].indexOf(4).should.equal(-1);
       }
     }
@@ -1837,12 +1849,12 @@ function ok(expr, msg) {
 
 suite('Array');
 
-test('#length', function() {
+test('#length', function () {
   var arr = [1, 2, 3];
   ok(arr.length == 3);
 });
 
-test('#indexOf()', function() {
+test('#indexOf()', function () {
   var arr = [1, 2, 3];
   ok(arr.indexOf(1) == 0);
   ok(arr.indexOf(2) == 1);
@@ -1851,7 +1863,7 @@ test('#indexOf()', function() {
 
 suite('String');
 
-test('#length', function() {
+test('#length', function () {
   ok('foo'.length == 3);
 });
 ```
@@ -1868,13 +1880,13 @@ var pre = require('mocha').before;
 var assertions = require('mocha').it;
 var assert = require('chai').assert;
 
-testCase('Array', function() {
-  pre(function() {
+testCase('Array', function () {
+  pre(function () {
     // ...
   });
 
-  testCase('#indexOf()', function() {
-    assertions('should return -1 when not present', function() {
+  testCase('#indexOf()', function () {
+    assertions('should return -1 when not present', function () {
       assert.equal([1, 2, 3].indexOf(4), -1);
     });
   });
@@ -1980,9 +1992,9 @@ The Doc reporter outputs a hierarchical HTML body representation of your tests.
 For example, suppose you have the following JavaScript:
 
 ```js
-describe('Array', function() {
-  describe('#indexOf()', function() {
-    it('should return -1 when the value is not present', function() {
+describe('Array', function () {
+  describe('#indexOf()', function () {
+    it('should return -1 when the value is not present', function () {
       [1, 2, 3].indexOf(5).should.equal(-1);
       [1, 2, 3].indexOf(0).should.equal(-1);
     });
diff --git a/lib/reporters/base.js b/lib/reporters/base.js
index 90f89c99bf..40b5996461 100644
--- a/lib/reporters/base.js
+++ b/lib/reporters/base.js
@@ -56,6 +56,11 @@ exports.useColors =
 
 exports.inlineDiffs = false;
 
+/**
+ * Truncate diffs longer than this value to avoid slow performance
+ */
+exports.maxDiffSize = 8192;
+
 /**
  * Default color map.
  */
@@ -188,18 +193,23 @@ function stringifyDiffObjs(err) {
  * @param {string} expected
  * @return {string} Diff
  */
+
 var generateDiff = (exports.generateDiff = function (actual, expected) {
   try {
-    const diffSize = 2048;
-    if (actual.length > diffSize) {
-      actual = actual.substring(0, diffSize) + ' ... Lines skipped';
+    var maxLen = exports.maxDiffSize;
+    var skipped = 0;
+    if (maxLen > 0) {
+      skipped = Math.max(actual.length - maxLen, expected.length - maxLen);
+      actual = actual.slice(0, maxLen);
+      expected = expected.slice(0, maxLen);
     }
-    if (expected.length > diffSize) {
-      expected = expected.substring(0, diffSize) + ' ... Lines skipped';
-    }
-    return exports.inlineDiffs
+    let result = exports.inlineDiffs
       ? inlineDiff(actual, expected)
       : unifiedDiff(actual, expected);
+    if (skipped > 0) {
+      result = `${result}\n      [mocha] output truncated to ${maxLen} characters, see "maxDiffSize" reporter-option\n`;
+    }
+    return result;
   } catch (err) {
     var msg =
       '\n      ' +
@@ -318,6 +328,12 @@ function Base(runner, options) {
   this.runner = runner;
   this.stats = runner.stats; // assigned so Reporters keep a closer reference
 
+  var maxDiffSizeOpt =
+    this.options.reporterOption && this.options.reporterOption.maxDiffSize;
+  if (maxDiffSizeOpt !== undefined && !isNaN(Number(maxDiffSizeOpt))) {
+    exports.maxDiffSize = Number(maxDiffSizeOpt);
+  }
+
   runner.on(EVENT_TEST_PASS, function (test) {
     if (test.duration > test.slow()) {
       test.speed = 'slow';
diff --git a/test/reporters/base.spec.js b/test/reporters/base.spec.js
index e13d1b2d7b..6b30b2ccc7 100644
--- a/test/reporters/base.spec.js
+++ b/test/reporters/base.spec.js
@@ -10,6 +10,9 @@ var Base = reporters.Base;
 var chaiExpect = chai.expect;
 var createElements = helpers.createElements;
 var makeTest = helpers.makeTest;
+var Mocha = require('../../');
+var Suite = Mocha.Suite;
+var Runner = Mocha.Runner;
 
 describe('Base reporter', function () {
   var stdout;
@@ -134,14 +137,13 @@ describe('Base reporter', function () {
     var inlineDiffsStub;
 
     beforeEach(function () {
-      inlineDiffsStub = sinon.stub(Base, 'inlineDiffs');
+      inlineDiffsStub = sinon.stub(Base, 'inlineDiffs').value(false);
     });
 
     it("should generate unified diffs if 'inlineDiffs' is false", function () {
       var actual = 'a foo unified diff';
       var expected = 'a bar unified diff';
 
-      inlineDiffsStub.value(false);
       var output = generateDiff(actual, expected);
 
       expect(
@@ -168,18 +170,44 @@ describe('Base reporter', function () {
     it("should truncate overly long 'actual' ", function () {
       var actual = '';
       var i = 0;
-      while (i++ < 120) {
+      while (i++ < 500) {
         actual += 'a foo unified diff ';
       }
       var expected = 'a bar unified diff';
 
-      inlineDiffsStub.value(false);
       var output = generateDiff(actual, expected);
 
-      expect(output, 'to match', / \.\.\. Lines skipped/);
+      expect(output, 'to match', /output truncated/);
     });
 
     it("should truncate overly long 'expected' ", function () {
+      var actual = 'a foo unified diff';
+      var expected = '';
+      var i = 0;
+      while (i++ < 500) {
+        expected += 'a bar unified diff ';
+      }
+
+      var output = generateDiff(actual, expected);
+
+      expect(output, 'to match', /output truncated/);
+    });
+
+    it("should not truncate overly long 'actual' if maxDiffSize=0", function () {
+      var actual = '';
+      var i = 0;
+      while (i++ < 120) {
+        actual += 'a bar unified diff ';
+      }
+      var expected = 'b foo unified diff';
+
+      sinon.stub(Base, 'maxDiffSize').value(0);
+      var output = generateDiff(actual, expected);
+
+      expect(output, 'not to match', /output truncated/);
+    });
+
+    it("should not truncate overly long 'expected' if maxDiffSize=0", function () {
       var actual = 'a foo unified diff';
       var expected = '';
       var i = 0;
@@ -187,10 +215,10 @@ describe('Base reporter', function () {
         expected += 'a bar unified diff ';
       }
 
-      inlineDiffsStub.value(false);
+      sinon.stub(Base, 'maxDiffSize').value(0);
       var output = generateDiff(actual, expected);
 
-      expect(output, 'to match', / \.\.\. Lines skipped/);
+      expect(output, 'not to match', /output truncated/);
     });
   });
 
@@ -278,6 +306,32 @@ describe('Base reporter', function () {
     });
   });
 
+  describe("when 'reporterOption.maxDiffSize' is provided", function () {
+    var origSize;
+
+    beforeEach(function () {
+      sinon.restore();
+      origSize = Base.maxDiffSize;
+    });
+
+    afterEach(function () {
+      Base.maxDiffSize = origSize;
+    });
+
+    it("should set 'Base.maxDiffSize' used for truncating diffs", function () {
+      var options = {
+        reporterOption: {
+          maxDiffSize: 4
+        }
+      };
+      var suite = new Suite('Dummy suite', 'root');
+      var runner = new Runner(suite);
+      // eslint-disable-next-line no-unused-vars
+      var base = new Base(runner, options);
+      expect(Base.maxDiffSize, 'to be', 4);
+    });
+  });
+
   it('should stringify objects', function () {
     var err = new Error('test');
     err.actual = {key: 'a1'};
@@ -457,6 +511,7 @@ describe('Base reporter', function () {
     var baseConsoleLog;
 
     beforeEach(function () {
+      sinon.restore();
       sinon.stub(console, 'log');
       baseConsoleLog = sinon.stub(Base, 'consoleLog');
     });
@@ -468,9 +523,5 @@ describe('Base reporter', function () {
       expect(baseConsoleLog, 'was called');
       expect(console.log, 'was not called');
     });
-
-    afterEach(function () {
-      sinon.restore();
-    });
   });
 });