From 2fc35fe36e01bdd93da8697789865db31e820a51 Mon Sep 17 00:00:00 2001
From: Max Freedom Pollard
 <272618364+MaxFreedomPollard@users.noreply.github.com>
Date: Mon, 14 Sep 2026 20:27:59 -0400
Subject: [PATCH] fix: apply the else branch when a schema has if and else but
 no then

buildValue only routed a schema to buildIfThenElse when both `if` and
`then` were present, so a schema that pairs `if` with `else` alone fell
through to ordinary serialization and every property declared only under
`else` was dropped from the output. The guard now also accepts `else`,
and buildIfThenElse treats `then` as optional: with no `then` the true
branch adds no keywords, so it serializes with the root schema. The
`then`-only and `then` plus `else` paths generate byte-identical code
to main.
---
 index.js                  | 30 +++++++++++++++++-------------
 test/if-then-else.test.js | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/index.js b/index.js
index 3a02da13..1b737eb5 100644
--- a/index.js
+++ b/index.js
@@ -1261,19 +1261,23 @@ function buildIfThenElse (context, location, input) {
   const ifSchemaRef = getValidatorSchemaRef(context, ifLocation)
   context.validatorSchemaRefs.add(ifSchemaRef)
 
-  const thenLocation = location.getPropertyLocation('then')
-  let thenMergedSchemaId = context.mergedSchemasIds.get(thenSchema)
-  let thenMergedLocation = null
-  if (thenMergedSchemaId) {
-    thenMergedLocation = getMergedLocation(context, thenMergedSchemaId)
-  } else {
-    thenMergedSchemaId = `__fjs_merged_${schemaIdCounter++}`
-    context.mergedSchemasIds.set(thenSchema, thenMergedSchemaId)
+  // `then` is optional: a schema may pair `if` with `else` alone. In that case
+  // the true branch adds no keywords, so it serializes with the root schema.
+  let thenMergedLocation = rootLocation
+  if (thenSchema !== undefined) {
+    const thenLocation = location.getPropertyLocation('then')
+    let thenMergedSchemaId = context.mergedSchemasIds.get(thenSchema)
+    if (thenMergedSchemaId) {
+      thenMergedLocation = getMergedLocation(context, thenMergedSchemaId)
+    } else {
+      thenMergedSchemaId = `__fjs_merged_${schemaIdCounter++}`
+      context.mergedSchemasIds.set(thenSchema, thenMergedSchemaId)
 
-    thenMergedLocation = mergeLocations(context, thenMergedSchemaId, [
-      rootLocation,
-      thenLocation
-    ])
+      thenMergedLocation = mergeLocations(context, thenMergedSchemaId, [
+        rootLocation,
+        thenLocation
+      ])
+    }
   }
 
   if (!elseSchema) {
@@ -1330,7 +1334,7 @@ function buildValue (context, location, input) {
     return buildOneOf(context, location, input)
   }
 
-  if (schema.if && schema.then) {
+  if (schema.if && (schema.then || schema.else)) {
     return buildIfThenElse(context, location, input)
   }
 
diff --git a/test/if-then-else.test.js b/test/if-then-else.test.js
index 9220a516..782a9c4f 100644
--- a/test/if-then-else.test.js
+++ b/test/if-then-else.test.js
@@ -650,3 +650,36 @@ test('if with a $ref that has sibling keywords is left unchanged', (t) => {
   const stringify = build(schema, { schema: externalSchema })
   t.assert.equal(stringify({ kind: 'foo', foo: 'ignored', bar: 'fallback' }), '{"bar":"fallback"}')
 })
+
+test('if/else without then applies the else branch', (t) => {
+  t.plan(4)
+
+  const schema = {
+    type: 'object',
+    properties: {
+      kind: { type: 'string' }
+    },
+    if: {
+      type: 'object',
+      properties: {
+        kind: { type: 'string', enum: ['foobar'] }
+      }
+    },
+    else: {
+      type: 'object',
+      properties: {
+        bar: { type: 'string' }
+      }
+    }
+  }
+
+  const stringify = build(schema)
+
+  const elseOutput = stringify({ kind: 'other', bar: 'value' })
+  t.assert.equal(elseOutput, '{"kind":"other","bar":"value"}')
+  t.assert.deepStrictEqual(JSON.parse(elseOutput), { kind: 'other', bar: 'value' })
+
+  const thenOutput = stringify({ kind: 'foobar', bar: 'value' })
+  t.assert.equal(thenOutput, '{"kind":"foobar"}')
+  t.assert.deepStrictEqual(JSON.parse(thenOutput), { kind: 'foobar' })
+})