Nested VError causes (pinojs#108)

* Failing test: support nested cause serialization for VError type errors

* Support VError type errors for nested serialization

Extracts evaluateCause from inside err-helpers and reuses it in error serialization to support nested causes where .cause is a function.

lib/err-helpers.js

@@ -12,23 +12,22 @@
 const getErrorCause = (err) => {
   if (!err) return
 
-  /** @type {unknown} */
-  // @ts-ignore
-  const cause = err.cause
+  const cause = evaluateCause(err.cause)
 
-  // VError / NError style causes
-  if (typeof cause === 'function') {
-    // @ts-ignore
-    const causeResult = err.cause()
+  return cause instanceof Error
+    ? cause
+    : undefined
+}
 
-    return causeResult instanceof Error
-      ? causeResult
-      : undefined
-  } else {
-    return cause instanceof Error
-      ? cause
-      : undefined
-  }
+/**
+ * @param {unknown|(()=>err)} cause
+ * @returns {Error|undefined}
+ */
+const evaluateCause = (cause) => {
+  // VError / NError style causes are functions
+  return typeof cause === 'function'
+    ? cause()
+    : cause
 }
 
 /**
@@ -108,6 +107,7 @@ const messageWithCauses = (err) => _messageWithCauses(err, new Set())
 
 module.exports = {
   getErrorCause,
+  evaluateCause,
   stackWithCauses,
   messageWithCauses
 }

lib/err.js

@@ -2,7 +2,7 @@
 
 module.exports = errSerializer
 
-const { messageWithCauses, stackWithCauses } = require('./err-helpers')
+const { messageWithCauses, stackWithCauses, evaluateCause } = require('./err-helpers')
 
 const { toString } = Object.prototype
 const seen = Symbol('circular-ref-tag')
@@ -66,7 +66,8 @@ function errSerializer (err) {
   }
 
   if (err.cause) {
-    _err.cause = errSerializer(err.cause)
+    const cause = evaluateCause(err.cause)
+    _err.cause = errSerializer(cause)
   }
 
   for (const key in err) {

test/err.test.js

@@ -218,3 +218,31 @@ test('serializes causes', function (t) {
   t.equal(serialized.cause.cause.message, 'baz')
   t.match(serialized.cause.cause.stack, /err\.test\.js:/)
 })
+
+test('serializes causes with VError support', function (t) {
+  t.plan(11)
+
+  // Fake VError-style setup
+  const err = Error('foo: bar')
+  err.cause = () => {
+    const err = Error('bar')
+    err.cause = Error('abc')
+    return err
+  }
+
+  const serialized = serializer(err)
+
+  t.equal(serialized.type, 'Error')
+  t.equal(serialized.message, 'foo: bar: abc') // message serialization already walks cause-chain
+  t.match(serialized.stack, /err\.test\.js:/)
+
+  t.ok(serialized.cause)
+  t.equal(serialized.cause.type, 'Error')
+  t.equal(serialized.cause.message, 'bar: abc')
+  t.match(serialized.cause.stack, /err\.test\.js:/)
+
+  t.ok(serialized.cause.cause)
+  t.equal(serialized.cause.cause.type, 'Error')
+  t.equal(serialized.cause.cause.message, 'abc')
+  t.match(serialized.cause.cause.stack, /err\.test\.js:/)
+})