Implement nullish coalescing operator (#3997)

JerryScript-DCO-1.0-Signed-off-by: bence gabor kis kisbg@inf.u-szeged.hu

jerry-core/vm/vm.c

@@ -2922,6 +2922,20 @@ vm_loop (vm_frame_ctx_t *frame_ctx_p) /**< frame context */
           ecma_fast_free_value (value);
           continue;
         }
+#if ENABLED (JERRY_ESNEXT)
+        case VM_OC_BRANCH_IF_NULLISH:
+        {
+          left_value = stack_top_p[-1];
+
+          if (!ecma_is_value_null (left_value) && !ecma_is_value_undefined (left_value))
+          {
+            byte_code_p = byte_code_start_p + branch_offset;
+            continue;
+          }
+          --stack_top_p;
+          continue;
+        }
+#endif /* ENABLED (JERRY_ESNEXT) */
         case VM_OC_PLUS:
         case VM_OC_MINUS:
         {

jerry-core/vm/vm.h

@@ -161,6 +161,9 @@ typedef enum
   VM_OC_ERROR,                   /**< error while the vm_loop is suspended */
 
   VM_OC_JUMP,                    /**< jump */
+#if ENABLED (JERRY_ESNEXT)
+  VM_OC_BRANCH_IF_NULLISH,       /** branch if undefined or null */
+#endif /* ENABLED (JERRY_ESNEXT) */
   VM_OC_BRANCH_IF_STRICT_EQUAL,  /**< branch if strict equal */
 
   /* These four opcodes must be in this order. */
@@ -296,6 +299,7 @@ typedef enum
 {
 #if !ENABLED (JERRY_ESNEXT)
   VM_OC_EXP = VM_OC_NONE,                     /**< exponentiation */
+  VM_OC_BRANCH_IF_NULLISH = VM_OC_NONE,       /** branch if undefined or null */
 #endif /* !ENABLED (JERRY_ESNEXT) */
 #if !ENABLED (JERRY_DEBUGGER)
   VM_OC_BREAKPOINT_ENABLED = VM_OC_NONE,      /**< enabled breakpoint for debugger is unused */