The following code snippet passes typescript checks

function foo(callback: (bool: boolean) => string) {
    callback(true);
}

type Interface1 = {
    func1: () => string;
};

const testFunction = (str: string = '') => str.toLowerCase();

const variable: Interface1 = { func1: testFunction };

// No typescript error, but will raise error on runtime
foo(variable.func1);

whereas the following statement throws a typescript error

foo(testFunction);

How could we catch this error at compile time?