Fix evaluateCause loosing access to this

lib/err-helpers.js

@@ -12,22 +12,22 @@
 const getErrorCause = (err) => {
   if (!err) return
 
-  const cause = evaluateCause(err.cause)
+  const cause = evaluateCause(err)
 
   return cause instanceof Error
     ? cause
     : undefined
 }
 
 /**
- * @param {unknown|(()=>err)} cause
+ * @param {Error|{ cause?: unknown|(()=>err)}} err
  * @returns {Error|undefined}
  */
-const evaluateCause = (cause) => {
+const evaluateCause = (err) => {
   // VError / NError style causes are functions
-  return typeof cause === 'function'
-    ? cause()
-    : cause
+  return typeof err.cause === 'function'
+    ? err.cause()
+    : err.cause
 }
 
 /**

lib/err.js

@@ -66,7 +66,7 @@ function errSerializer (err) {
   }
 
   if (err.cause) {
-    const cause = evaluateCause(err.cause)
+    const cause = evaluateCause(err)
     _err.cause = errSerializer(cause)
   }
 

test/err.test.js

@@ -223,16 +223,26 @@ 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
+  class VError extends Error {
+    constructor (message, cause) {
+      super(message + ': ' + cause.message)
+      this._cause = cause
+    }
+
+    // Ensure `cause` refers `this` (see https://github.com/pinojs/pino-std-serializers/pull/109)
+    cause () {
+      return this._cause
+    }
   }
 
+  const causeErr = Error('bar')
+  causeErr.cause = Error('abc')
+
+  const err = new VError('foo', causeErr)
+
   const serialized = serializer(err)
 
-  t.equal(serialized.type, 'Error')
+  t.equal(serialized.type, 'VError')
   t.equal(serialized.message, 'foo: bar: abc') // message serialization already walks cause-chain
   t.match(serialized.stack, /err\.test\.js:/)