Allow modifying next.value

index.js

@@ -768,7 +768,7 @@ class Batch {
           if (cas && !(await cas((await node.getKeyNode(mid)).final(encoding), newNode))) return this._unlockMaybe()
 
           node.setKey(mid, target)
-          return this._append(root, seq, key, value)
+          return this._append(root, seq, key, newNode.value)
         }
 
         if (c < 0) e = mid
@@ -798,7 +798,7 @@ class Batch {
       }
     }
 
-    return this._append(root, seq, key, value)
+    return this._append(root, seq, key, enc(encoding.value, newNode.value))
   }
 
   async del (key, opts) {

test/cas.js

@@ -2,6 +2,62 @@ const test = require('brittle')
 const b4a = require('b4a')
 const { create } = require('./helpers')
 
+test('cas - swap with a new value', async function (t) {
+  const db = create({ valueEncoding: 'json' })
+
+  await db.put('/a', 0, {
+    cas: function (prev, next) {
+      t.is(prev, null)
+      t.alike(next, { seq: 1, key: '/a', value: 0 })
+      return next
+    }
+  })
+
+  t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 0 })
+
+  await db.put('/a', 99, {
+    cas: function (prev, next) {
+      t.alike(prev, { seq: 1, key: '/a', value: 0 })
+      t.alike(next, { seq: 2, key: '/a', value: 99 })
+      next.value = prev.value + 1 // Overwrites so it's not 99 anymore
+      return next
+    }
+  })
+
+  t.alike(await db.get('/a'), { seq: 2, key: '/a', value: 1 })
+
+  await db.put('/a', 99, {
+    cas: function (prev, next) {
+      t.alike(prev, { seq: 2, key: '/a', value: 1 })
+      t.alike(next, { seq: 3, key: '/a', value: 99 })
+      next.value = prev.value + 1
+      return next
+    }
+  })
+
+  t.alike(await db.get('/a'), { seq: 3, key: '/a', value: 2 })
+})
+
+test('cas - should not swap', async function (t) {
+  t.plan(4)
+
+  const db = create({ valueEncoding: 'json' })
+
+  await db.put('/a', 1)
+
+  t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 1 })
+
+  await db.put('/a', 2, {
+    cas: function (prev, next) {
+      t.alike(prev, { seq: 1, key: '/a', value: 1 })
+      t.alike(next, { seq: 2, key: '/a', value: 2 })
+      return null
+    }
+  })
+
+  t.alike(await db.get('/a'), { seq: 1, key: '/a', value: 1 })
+})
+
 test('cas is called when prev does not exists', async function (t) {
   t.plan(6)