does React restrict usage of undeclared variables?

Question

Code snippet:

class Box extends React.Component{
  render() {
    params = new URLSearchParams(this.props.location.search);
    abc = params.get('abc');
    console.log(params);

Error via "Create React App":

Why am I getting an error here?

Answer 1

In non-strict mode, an assignment to an undeclared symbol is implicitly treated as creating a global variable. In strict mode, that's an error.

I just checked Reacts compiled code and it has "use strict"; at the top. So yes, React restricts undeclared variables.