How to Config babel.config to extend classes

I am building a react-component.
I am using some Es6 class like 
`class MyClass extends MainClass {
   // ...
}
let instance = new MyClass() `
Code is complied successfully 
but when i run it.
It throws an **error Uncaught TypeError: Class constructor MainClass cannot be invoked without 'new'**
I am using  "webpack": "4.43.0",  "npm": "4.43.0", node:14.4.0 and nwb : 0.25.2.

