What does these functions do in javascript?

1

what does unshift do?

countArray.unshift(n);

what does splice do?

function countdown(n) {
  if (n < 1) {
    return [];
  } else {
    const arr = countdown(n - 1);
    arr.splice(0, 0, n);
    return arr;
  }
}

and concat in this example?

function countdown(n){
   return n < 1 ? [] : [n].concat(countdown(n - 1));
}
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