Fix == / != to match JavaScript abstract equality for null

src/JWadhams/JsonLogic.php

@@ -60,12 +60,22 @@ public static function apply($logic = [], $data = [])
 
         $operators = [
             '==' => function ($a, $b) {
+                // Match JavaScript's abstract equality for null: in JS `null` is
+                // loosely-equal only to `null`/`undefined`, never to 0, false or "".
+                // PHP's native `==` coerces null to those, so guard it here to keep
+                // rules evaluating identically to json-logic-js.
+                if ($a === null || $b === null) {
+                    return $a === null && $b === null;
+                }
                 return $a == $b;
             },
             '===' => function ($a, $b) {
                 return $a === $b;
             },
             '!=' => function ($a, $b) {
+                if ($a === null || $b === null) {
+                    return !($a === null && $b === null);
+                }
                 return $a != $b;
             },
             '!==' => function ($a, $b) {

tests/NullLooseEqualityTest.php

@@ -0,0 +1,56 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * `==` / `!=` must match JavaScript's abstract equality for `null`, so a rule
+ * evaluates identically in json-logic-js (front end) and this library (back end).
+ *
+ * In JS, `null` is loosely-equal only to `null`/`undefined` — never to 0, false
+ * or "". PHP's native `==` coerces `null` to those, so `null == 0` wrongly
+ * returned true here while json-logic-js returns false.
+ */
+class NullLooseEqualityTest extends TestCase
+{
+    /**
+     * @dataProvider nullEqualityProvider
+     */
+    public function testNullEquality($logic, $expected)
+    {
+        $this->assertSame(
+            $expected,
+            JWadhams\JsonLogic::apply($logic),
+            "JsonLogic::apply(" . json_encode($logic) . ") === " . json_encode($expected)
+        );
+    }
+
+    public function nullEqualityProvider()
+    {
+        return [
+            // null is loosely-equal only to null (JS: null == undefined too, but
+            // JSON has no undefined — a missing var resolves to null).
+            'null == null'      => [['==' => [null, null]], true],
+            'null == 0'         => [['==' => [null, 0]], false],
+            'null == 0.0'       => [['==' => [null, 0.0]], false],
+            'null == false'     => [['==' => [null, false]], false],
+            'null == ""'        => [['==' => [null, '']], false],
+            'null == "0"'       => [['==' => [null, '0']], false],
+            'null == []'        => [['==' => [null, []]], false],
+            '0 == null'         => [['==' => [0, null]], false],
+            'false == null'     => [['==' => [false, null]], false],
+
+            // != is the negation.
+            'null != null'      => [['!=' => [null, null]], false],
+            'null != 0'         => [['!=' => [null, 0]], true],
+            'null != false'     => [['!=' => [null, false]], true],
+            '0 != null'         => [['!=' => [0, null]], true],
+
+            // Non-null coercion is unchanged (matches JS already).
+            '"0" == 0'          => [['==' => ['0', 0]], true],
+            '1 == 1'            => [['==' => [1, 1]], true],
+            '0 == 0'            => [['==' => [0, 0]], true],
+            'false == 0'        => [['==' => [false, 0]], true],
+            '1 != 2'            => [['!=' => [1, 2]], true],
+        ];
+    }
+}