Append the cause stacks to the main stack trace

It would be great to get the full error stack chain for errors with causes, especially as all current browsers and Node.js >=16 supports it, see eg https://v8.dev/features/error-cause and https://dev.to/voxpelli/pony-cause-1-0-error-causes-2l2o

Eg. `pino` merged support for this as well: pinojs/pino-std-serializers#78

lib/reporters/base.js

@@ -221,6 +221,23 @@ var generateDiff = (exports.generateDiff = function (actual, expected) {
   }
 });
 
+/**
+ * This function dumps long stack traces for exceptions having
+ *  an Error cause() method
+ *
+ * @public
+ * @memberof Mocha.reporters.Base
+ * @param {Error} ex
+ * @return {string}
+ */
+exports.getFullErrorStack = function (err, message) {
+  var ret = err.stack || message || '';
+  if (err.cause) {
+    ret += '\n' + '   ' + 'Caused by: ' + exports.getFullErrorStack(err.cause);
+  }
+  return ret;
+};
+
 /**
  * Outputs the given `failures` as a list.
  *
@@ -260,7 +277,7 @@ exports.list = function (failures) {
     } else {
       message = '';
     }
-    var stack = err.stack || message;
+    var stack = exports.getFullErrorStack(err, message);
     var index = message ? stack.indexOf(message) : -1;
 
     if (index === -1) {

lib/runner.js

@@ -450,6 +450,22 @@ Runner.prototype.fail = function (test, err, force) {
     // some environments do not take kindly to monkeying with the stack
   }
 
+  // Support for error causes
+  try {
+    let nextErr = err;
+
+    while (nextErr.cause && nextErr.cause.stack) {
+      nextErr = nextErr.cause;
+
+      err.stack +=
+        '\n' + (this.fullStackTrace || !nextErr.stack)
+          ? nextErr.stack
+          : stackFilter(nextErr.stack);
+    }
+  } catch (ignore) {
+    // some environments do not take kindly to monkeying with the stack
+  }
+
   this.emit(constants.EVENT_TEST_FAIL, test, err);
 };
 

test/reporters/base.spec.js

@@ -477,6 +477,32 @@ describe('Base reporter', function () {
     expect(errOut, 'to be', '1) test title:');
   });
 
+  it('should append any error cause trail to stack traces', function () {
+    var err = {
+      message: 'Error',
+      stack: 'Error\nfoo\nbar',
+      showDiff: false,
+      cause: {
+        message: 'Cause1',
+        stack: 'Cause1\nbar\nfoo',
+        showDiff: false,
+        cause: {
+          message: 'Cause2',
+          stack: 'Cause2\nabc\nxyz',
+          showDiff: false
+        }
+      }
+    };
+    var test = makeTest(err);
+
+    Base.list([test]);
+
+    var errOut = stdout.join('\n').trim();
+    errOut.should.equal(
+      '1) test title:\n     Error\n  foo\n  bar\n     Caused by: Cause Stack'
+    );
+  });
+
   it('should not modify stack if it does not contain message', function () {
     var err = {
       message: 'Error',