const arr = [1, [2, 3], [4, [5, 6]], 7, [8, [9, [10]]]];
function getArray(input){
let result = [];
for(let item of input){
if(Array.isArray(item)){
result = result.concat(getArray(item));
}else{
result.push(item)
}
}
return result;
}
console.log(getArray(arr));
// output : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]



