fix(slice): explicit zero end no longer returns the whole array (#265)

A slice with an explicit `0` end (e.g. `$[:0]`, `$[2:0]`) returned the
entire array instead of an empty result. `end` was computed with
`(parts[1] && Number.parseInt(parts[1])) || len`, so a parsed end of `0`
is falsy and got replaced by the array length. Treat an omitted end
(empty string) as the length and parse any explicit value, including 0.

Author: spokodev

src/jsonpath.js

@@ -608,7 +608,7 @@ JSONPath.prototype._slice = function (
     const len = val.length, parts = loc.split(':'),
         step = (parts[2] && Number.parseInt(parts[2])) || 1;
     let start = (parts[0] && Number.parseInt(parts[0])) || 0,
-        end = (parts[1] && Number.parseInt(parts[1])) || len;
+        end = parts[1] ? Number.parseInt(parts[1]) : len;
     start = (start < 0) ? Math.max(0, start + len) : Math.min(len, start);
     end = (end < 0) ? Math.max(0, end + len) : Math.min(len, end);
     const ret = [];

test/test.slice.js

@@ -42,4 +42,25 @@ describe('JSONPath - slice', function () {
         });
         assert.deepEqual(result, expected);
     });
+
+    it('should return empty array for a slice with an explicit zero end', function () {
+        const jsonWithChildren = {
+            "name": "root",
+            "children": [
+                {a: 1}, {a: 2}, {a: 3}, {a: 4}, {a: 5}, {a: 6}
+            ]
+        };
+        assert.deepEqual(jsonpath({
+            json: jsonWithChildren,
+            path: '$.children[:0]'
+        }), []);
+        assert.deepEqual(jsonpath({
+            json: jsonWithChildren,
+            path: '$.children[2:0]'
+        }), []);
+        assert.deepEqual(jsonpath({
+            json: jsonWithChildren,
+            path: '$.children[0:0]'
+        }), []);
+    });
 });