Fixed sum() and join() to return 0 and an empty string respectively for empty arrays

CHANGELOG.md

@@ -12,6 +12,7 @@
 * Changed sort(), sort_by(), max(), min(), max_by() and min_by() to order strings by code point.
 * Fixed max_by() and min_by() to error on mixed-type keys instead of returning arbitrary elements.
 * Fixed max() returning null or erroring when the first array element is falsy, e.g. max([0, 1]).
+* Fixed sum() and join() to return 0 and an empty string respectively for empty arrays.
 * Fixed 0.0 to be truthy in filters and logical operators, like every other number.
 * Fixed the compiled runtime to apply JMESPath truthiness to || and &&.
 * Fixed @(foo), foo[-] and oversized index literals to throw syntax errors.

src/FnDispatcher.php

@@ -106,7 +106,7 @@ private function fn_join(array $args)
         $fn = function ($a, $b, $i) use ($args) {
             return $i ? ($a . $args[0] . $b) : $b;
         };
-        return $this->reduce('join:0', $args[1], ['string'], $fn);
+        return $args[1] ? $this->reduce('join:0', $args[1], ['string'], $fn) : '';
     }
 
     private function fn_keys(array $args)
@@ -203,7 +203,7 @@ private function fn_sum(array $args)
         $fn = function ($a, $b) {
             return Utils::add($a, $b);
         };
-        return $this->reduce('sum:0', $args[0], ['number'], $fn);
+        return $args[0] ? $this->reduce('sum:0', $args[0], ['number'], $fn) : 0;
     }
 
     private function fn_sort(array $args)

tests/FnDispatcherTest.php

@@ -50,6 +50,22 @@ public function testContainsUsesJsonSemantics(): void
         $this->assertFalse($fn('contains', ['foobar', 123]));
     }
 
+    public function testSumOfEmptyArrayIsZero(): void
+    {
+        $fn = new FnDispatcher();
+
+        $this->assertSame(0, $fn('sum', [[]]));
+        $this->assertSame(3, $fn('sum', [[1, 2]]));
+    }
+
+    public function testJoinOfEmptyArrayIsEmptyString(): void
+    {
+        $fn = new FnDispatcher();
+
+        $this->assertSame('', $fn('join', ['|', []]));
+        $this->assertSame('a|b', $fn('join', ['|', ['a', 'b']]));
+    }
+
     public function testMaxAndMinCompareStringsByCodePoint(): void
     {
         $fn = new FnDispatcher();