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

lib/reporters/base.js

@@ -155,6 +155,10 @@ exports.cursor = {
   }
 };
 
+function showDiff(err) {
+  return err && err.showDiff !== false && sameType(err.actual, err.expected) && err.expected !== undefined;
+}
+
 /**
  * Output the given `failures` as a list.
  *
@@ -183,8 +187,6 @@ exports.list = function (failures) {
     }
     var stack = err.stack || message;
     var index = message ? stack.indexOf(message) : -1;
-    var actual = err.actual;
-    var expected = err.expected;
     var escape = true;
 
     if (index === -1) {
@@ -201,13 +203,8 @@ exports.list = function (failures) {
       msg = 'Uncaught ' + msg;
     }
     // explicitly show diff
-    if (err.showDiff !== false && sameType(actual, expected) && expected !== undefined) {
+    if (showDiff(err)) {
       escape = false;
-      if (!(utils.isString(actual) && utils.isString(expected))) {
-        err.actual = actual = utils.stringify(actual);
-        err.expected = expected = utils.stringify(expected);
-      }
-
       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);
@@ -292,6 +289,12 @@ function Base (runner) {
   runner.on('fail', function (test, err) {
     stats.failures = stats.failures || 0;
     stats.failures++;
+    if (showDiff(err)) {
+      if (!utils.isString(err.actual) || !utils.isString(err.expected)) {
+        err.actual = utils.stringify(err.actual);
+        err.expected = utils.stringify(err.expected);
+      }
+    }
     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 () {