To use JavaScript for Internet of things (IoT), it is essential to reduce the size of the virtual machine (VM). Although operators in JavaScript is polymorphic, many application give only a limited datatypes of operands to operators. Thus, customizing VM so that the VM has only implementations of operators for such datatypes can reduce the size of the VM. In this research, we determine the set of operand datatypes that are possibly given to each operator for individual applications. Because JavaScript is a dynamically typed programming language, it is difficult to infer types accurately. Instead, as a practical approach, we obtain type information from executions of unit tests of the application. We implemented a system to record datatypes that are given to each operator in executions of unit tests for Jasmine. The system does not record operands of operators used in the execution framework or unit tests, but only records those used in the application in question selectively. We applied this system to two realistic applications and found it determined type information with high accuracy.