Browser compability fixes

Plus remove console.error statment
Plus add tests for option handling

lib/reporters/base.js

@@ -196,11 +196,13 @@ function stringifyDiffObjs(err) {
 
 var generateDiff = (exports.generateDiff = function (actual, expected) {
   try {
-    const maxLen = exports.maxDiffSize || Number.MAX_SAFE_INTEGER;
-    const skipped = Math.max(actual.length - maxLen, expected.length - maxLen);
-
-    actual = actual.slice(0, maxLen);
-    expected = expected.slice(0, maxLen);
+    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);
+    }
     let result = exports.inlineDiffs
       ? inlineDiff(actual, expected)
       : unifiedDiff(actual, expected);
@@ -328,8 +330,7 @@ function Base(runner, options) {
 
   var maxDiffSizeOpt =
     this.options.reporterOption && this.options.reporterOption.maxDiffSize;
-  console.error();
-  if (maxDiffSizeOpt !== undefined && !Number.isNaN(Number(maxDiffSizeOpt))) {
+  if (maxDiffSizeOpt !== undefined && !isNaN(Number(maxDiffSizeOpt))) {
     exports.maxDiffSize = Number(maxDiffSizeOpt);
   }
 

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;
@@ -505,3 +508,18 @@ describe('Base reporter', function () {
     });
   });
 });
+describe('when "reporterOption.maxDiffSize" is provided', function () {
+  it('should be the effective value used for tuncating diffs', function () {
+    var options = {
+      reporterOption: {
+        maxDiffSize: 4
+      }
+    };
+    var mocha = new Mocha();
+    var suite = new Suite('Dummy suite', 'root');
+    var runner = new Runner(suite);
+    // eslint-disable-next-line no-unused-vars
+    var mochaReporter = new mocha._reporter(runner, options);
+    expect(Base.maxDiffSize, 'to be', 4);
+  });
+});