[js] Implement nqp::existspos.

Author: pmurias

src/vm/js/Operations.nqp

@@ -920,7 +920,7 @@ class QAST::OperationsJS {
     add_simple_op('existskey', $T_BOOL, [$T_OBJ, $T_STR], :method_call);
     add_simple_op('deletekey', $T_OBJ, [$T_OBJ, $T_STR], :method_call, :side_effects);
 
-    add_simple_op('existspos', $T_BOOL, [$T_OBJ, $T_INT]);
+    add_simple_op('existspos', $T_BOOL, [$T_OBJ, $T_INT], :method_call);
 
     for <ceil floor abs log sqrt exp sin acos cos atan tan asin sinh cosh tanh> -> $func {
         add_simple_op($func ~ '_n', $T_NUM, [$T_NUM], sub ($arg) {"Math.$func($arg)"});

src/vm/js/nqp-runtime/core.js

@@ -191,12 +191,6 @@ exports.op.ishash = function(obj) {
   return obj instanceof Hash || (obj._STable && obj._STable.REPR instanceof reprs.VMHash) ? 1 : 0;
 };
 
-op.existspos = function(array, idx) {
-  if (array.$$existspos) return array.$$existspos(idx);
-  if (idx < 0) idx += array.length;
-  return array.hasOwnProperty(idx) ? 1 : 0;
-};
-
 op.create = function(obj) {
   return obj._STable.REPR.allocate(obj._STable);
 };

src/vm/js/nqp-runtime/reprs.js

@@ -1058,7 +1058,7 @@ class VMArray extends REPR {
 
       $$existspos(index) {
         if (index < 0) index += this.array.length;
-        return this.array.hasOwnProperty(index) ? 1 : 0;
+        return (this.array[index] === Null || this.array[index] === undefined) ? 0 : 1;
       }
 
       $$setelems(elems) {