How to call a function within Frame?

Question

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

Inside Javascript I have a variable (Z = 2.5, only one example of same value) of type number. I want to take that variable and move on to the function that’s inside a iframe.

Within the iframe this function only takes and returns a value.

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

Within the iframe still have some calculations that will be made with this value.

How to call this function through Javascript or HTML?

I’ve tried using var index = window.Parent; index.testePai(); and it didn’t work.