Function that flattens an array of arbitrarily nested arrays of integers into a flat array of integers

deflat array

function flattenArrayOfArrays(a, r){
    if(!r){ r = []}
    for(var i=0; i<a.length; i++){
        if(a[i].constructor == Array){
console.log(r)
            flattenArrayOfArrays(a[i], r);
        }else{
            r.push(a[i]);
        }
    }
    return r;
}
flattenArrayOfArrays([[1,2,[3]],4])