Array flattening

function flatten(input){
    const out = []; 
    
    function recurse(a){
        // This captures all iterators, not just arrays.
        if(a[Symbol.iterator]) {
            for(let v of a){
                recurse(v);
            }
        } 
        else {
            out.push(a);
        } 
    }
    recurse(input); 
    return out;
};