var arr= [[1,2,3,[4,5]],6]
var arr1 = []
function OneD(arr) {
for(var i = 0;i<arr.length;i++){
if(arr[i] instanceof Array){
OneD(arr[i])
}else{
arr1.push(arr[i])
}
}
}
OneD(arr)
console.log(arr1);