Edit in JSFiddle

var arrNum = [1,2,3,4,5],
    r = document.getElementById('result'),
    
    t = [], // set a global variable to update.
    
    double = function(n, index){ // use the index argument 
        t[index] = n * 2; 
        return t;        
    };
    
    arrNum.forEach( double ); // get rid of foo entirely.

r.innerHTML = t; // update with the new value of t

<div id="result"></div>