I am trying to flatten the array regardless of the level. I run it in debugger.it is working well until the last curly bracket

Question

function flatten(arg){
  const d=[];
  
if(!Array.isArray(arg)){
  d.push(arg);
  
}else{
flatten(arg.reduce((a,b)=>a.concat(b))); 
}
return d;
}

console.log(flatten