How to call a function inside the Frame?

I want to call a function that I have inside a iframe . I program in Ionic and wanted to know how I do this through JavaScript or HTML.

Within JavaScript I have a variable ( Z = 2.5 , just an example of the same value) of type number . I want to get this variable and move to the function that is inside a iframe .

Within iframe this function only takes and returns a value.

function pegavalor(){
    var url = 2.5; 
    var ff = url; 
    return ff; 
}

Within iframe you still have some calculations that will be done with that value.

How do I call this function through JavaScript or HTML?

I already tried to use var index = window.parent; index.testePai ();  and it did not work.