//nested array
var a=[[1,2,[3]],4];
//flattened result array
var fa=[];
//flatten function
function flatten(v){
    for(var i=0;i< v.length;i++){
        var o=v[i];
        //check if array value is instance of Array if so call flatten - recursive call
        if(o instanceof Array){
            flatten(o);
        }else{
            fa.push(o);
        }
    }
}
flatten(a);