Adapt stackWithCauses() to node.js output

Fixes #22
Related to: staltz/clarify-error#1 and nodejs/node#41002

Author: voxpelli

index.js

@@ -103,11 +103,17 @@ const _stackWithCauses = (err, seen) => {
 
   const cause = getErrorCause(err);
 
-  // TODO: Follow up in https://github.com/nodejs/node/issues/38725#issuecomment-920309092 on how to log stuff
-
   if (cause) {
     seen.add(err);
     return (stack + '\ncaused by: ' + _stackWithCauses(cause, seen));
+  } else if (
+    // @ts-ignore
+    err.cause
+  ) {
+    return (stack + '\ncaused by: ' + JSON.stringify(
+      // @ts-ignore
+      err.cause
+    ));
   } else {
     return stack;
   }

test/stack.spec.js

@@ -92,4 +92,37 @@ describe('stackWithCauses()', () => {
     const result = stackWithCauses(err);
     result.should.equal('xyz789\ncaused by: abc123\ncaused by: xyz789\ncauses have become circular...');
   });
+
+  it('should append non-Error string causes to the end of the cause trail', () => {
+    const cause = 'string cause';
+
+    const err = new ErrorWithCause('foo', { cause });
+    err.stack = 'xyz789';
+
+    const result = stackWithCauses(err);
+    should.exist(result);
+    result.should.equal('xyz789\ncaused by: "string cause"');
+  });
+
+  it('should append non-Error number causes to the end of the cause trail', () => {
+    const cause = 123;
+
+    const err = new ErrorWithCause('foo', { cause });
+    err.stack = 'xyz789';
+
+    const result = stackWithCauses(err);
+    should.exist(result);
+    result.should.equal('xyz789\ncaused by: 123');
+  });
+
+  it('should append non-Error object causes to the end of the cause trail', () => {
+    const cause = { name: 'TypeError', message: 'foo' };
+
+    const err = new ErrorWithCause('foo', { cause });
+    err.stack = 'xyz789';
+
+    const result = stackWithCauses(err);
+    should.exist(result);
+    result.should.equal('xyz789\ncaused by: {"name":"TypeError","message":"foo"}');
+  });
 });