Type ‘undefined’ cannot be used as index type

The compiler just tells you that this.state.sortCol might not have a value because you have the strictNullChecks flag on.

You can first check for its existence:

const fName = this.state.sortCol != null ? 
WidgetTableCols[this.state.sortCol].toLocaleLowerCase() : null;

Which will remove the error (but you will then need to deal with the fact that fName can be null).

You can also use the Non-null assertion operator:

const fName: string = 
WidgetTableCols[this.state.sortCol!].toLocaleLowerCase();

If you’re sure that it exists.