code: function isParameterAssigned(symbol) {
              var func = ts.getRootDeclaration(symbol.valueDeclaration).parent;
              var links = getNodeLinks(func);
              if (!(links.flags & 4194304 /* AssignmentsMarked */)) {
                  links.flags |= 4194304 /* AssignmentsMarked */;
                  if (!hasParentWithAssignmentsMarked(func)) {
                      markParameterAssignments(func);
                  }
              }
              return symbol.isAssigned || false;
          }
docs: Check if a parameter is assigned anywhere within its declaring function.