Flatten array of nested arrays Javascript

flatten.js

function flatten(arr) {
  a = []
  arr.forEach((s) => {
    if(Array.isArray(s)){
      a = a.concat(flatten(s))
    }else{
      a.push(s)
    }
  })
  
  return a
}


//Use case 1
[1,2,3,4]
//Use case 2
[1,[2,3],4]
//use case 3
[1,[2,3],[4,5,[6,7],8],9,10]