From 84ff329e14d8d07638f3a78af32cdbfbb349c6c7 Mon Sep 17 00:00:00 2001
From: Alec Larson <1925840+aleclarson@users.noreply.github.com>
Date: Fri, 8 Mar 2024 18:18:53 -0500
Subject: [PATCH] fix: surface deepest error from failed union validation
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The "Expected union value" error message is practically useless, so I think it‘s a better DX to surface the deepest validation error (based on property path).
---
 src/errors/errors.ts | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/errors/errors.ts b/src/errors/errors.ts
index f81de1a6..67bd2274 100644
--- a/src/errors/errors.ts
+++ b/src/errors/errors.ts
@@ -513,14 +513,16 @@ function* FromUndefined(schema: TUndefined, references: TSchema[], path: string,
   if (!IsUndefined(value)) yield Create(ValueErrorType.Undefined, schema, path, value)
 }
 function* FromUnion(schema: TUnion, references: TSchema[], path: string, value: any): IterableIterator<ValueError> {
-  let count = 0
+  let unionErrors: ValueError[] | undefined
   for (const subschema of schema.anyOf) {
     const errors = [...Visit(subschema, references, path, value)]
     if (errors.length === 0) return // matched
-    count += errors.length
+    if (unionErrors) {for (const error of errors) unionErrors.push(error)}
+    else {unionErrors = errors}
   }
-  if (count > 0) {
-    yield Create(ValueErrorType.Union, schema, path, value)
+  if (unionErrors) {
+    const pathDepth = (error: ValueError) => error.path.split('/').length
+    yield unionErrors.reduce((deepestError, error) => (!deepestError || pathDepth(error) > pathDepth(deepestError) ? error : deepestError))
   }
 }
 function* FromUint8Array(schema: TUint8Array, references: TSchema[], path: string, value: any): IterableIterator<ValueError> {