fix inaccurate diff output due to post-assertion object mutation (mochajs#3075)

* have Base generate the strings for error messages immediately instead of delaying until epilogue, the reason for this is that it is possible to reference objects in errors that could be mutated after the error occurs, causing the printed error message to be misleading

* re-add stringify during list to appease tests, open for discussion if this is correct behavior

Author: abrady0

lib/reporters/base.js

@@ -155,6 +155,17 @@ exports.cursor = {
   }
 };
 
+function showDiff (err) {
+  return err && err.showDiff !== false && sameType(err.actual, err.expected) && err.expected !== undefined;
+}
+
+function stringifyDiffObjs (err) {
+  if (!utils.isString(err.actual) || !utils.isString(err.expected)) {
+    err.actual = utils.stringify(err.actual);
+    err.expected = utils.stringify(err.expected);
+  }
+}
+
 /**
  * Output the given `failures` as a list.
  *
@@ -183,8 +194,6 @@ exports.list = function (failures) {
     }
     var stack = err.stack || message;
     var index = message ? stack.indexOf(message) : -1;
-    var actual = err.actual;
-    var expected = err.expected;
 
     if (index === -1) {
       msg = message;
@@ -200,12 +209,8 @@ exports.list = function (failures) {
       msg = 'Uncaught ' + msg;
     }
     // explicitly show diff
-    if (err.showDiff !== false && sameType(actual, expected) && expected !== undefined) {
-      if (!(utils.isString(actual) && utils.isString(expected))) {
-        err.actual = actual = utils.stringify(actual);
-        err.expected = expected = utils.stringify(expected);
-      }
-
+    if (showDiff(err)) {
+      stringifyDiffObjs(err);
       fmt = color('error title', '  %s) %s:\n%s') + color('error stack', '\n%s\n');
       var match = message.match(/^([^:]+): expected/);
       msg = '\n      ' + color('error message', match ? match[1] : msg);
@@ -290,6 +295,9 @@ function Base (runner) {
   runner.on('fail', function (test, err) {
     stats.failures = stats.failures || 0;
     stats.failures++;
+    if (showDiff(err)) {
+      stringifyDiffObjs(err);
+    }
     test.err = err;
     failures.push(test);
   });

test/reporters/list.spec.js

@@ -192,6 +192,28 @@ describe('List reporter', function () {
 
       Base.cursor = cachedCursor;
     });
+    it('should immediately construct fail strings', function () {
+      var actual = { a: 'actual' };
+      var expected = { a: 'expected' };
+      var test = {};
+      var checked = false;
+      var err;
+      runner.on = function (event, callback) {
+        if (!checked && event === 'fail') {
+          err = new Error('fake failure object with actual/expected');
+          err.actual = actual;
+          err.expected = expected;
+          err.showDiff = true;
+          callback(test, err);
+          checked = true;
+        }
+      };
+      List.call({epilogue: function () {}}, runner);
+
+      process.stdout.write = stdoutWrite;
+      expect(typeof err.actual).to.equal('string');
+      expect(typeof err.expected).to.equal('string');
+    });
   });
 
   describe('on end', function () {