JSFiddle - React, Tailwind, and code Playground

JavaScript

var x = 10,
    y = 20;

function f1(y){
    x = x + y;
      
    function f2(){
        y = y + 5;        
        return new Promise(c => c([x, y]));
    }
  
    return f2();
} 

f1(x).then((z) => {
		alert(z[0]); //20
    alert(z[1]); //15
});