When you encounter an error saying ‘ReactDOM’ is not defined, it means that your application is can’t find the variable
ReactDOM because it hasn’t been initialized yet.
Most likely, you forgot to import it from the package
import ReactDOM from 'react-dom'
Or if you use ES5:
const ReactDOM = require('react-dom')
If you have the import but still got the error, check the letter case of both your import and your code that calls the variable. Make sure it’s exactly the same case. In the example below, the variable is called as
ReactDom while the import is defined as
ReactDOM. This will not work:
import ReactDOM from 'react-dom' ReactDom.render()