Probleme in recursion JavaScript

I'm a beginner in JavaScript and would like to know what is the point of assign countdown(n - 1) to countArray and why we don't use push() instead of unshift () explanation, please

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function countdown(n){
  if (n < 1) {
    return [];
  } else {
    var countArray=countdown(n - 1);
    countArray.unshift(n);
    return countArray;
  }
}

console.log(countdown(5)); // [5, 4, 3, 2, 1]

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So here you give a number like 5, so it will create an array with reverse order like [5,4,3,2,1].

Unshift is basically to put the element at the first of the array

Let us go by one by one step

first it will check 5<1, no so it goes to else condition and creates a function call with countdown(4),

so like wise it will create function stack till 1 countdown(1) because when it reaches 0 it will return an empty array since if condition matches

so then it will resolve from top of the stacktrace,

stacktrace will be

• countdown(1)
• countdown(2)
• countdown(3)
• countdown(4)
• countdown(5)

so the empty array has been returned to that empty array countdown(1) will execute and unshift to the first position

now array will be [1]

next it will execute the countdown(2) and unshift means to the first index it will add so the 1 gets moved to the second position

now array will be [2,1]

like wise

it reaches countdown(5)

and array will be [5,4,3,2,1]

thats it

countdown(n - 1) calls the countdown function with the value of ndecremented by one. So for example, your if first call is countdown(5), when the line countdown(n - 1) is encountered, countdown is called with the value 4 - (countdown(4)).

This continues until the value of n is less than 0 which is when an empty array is returned from the function.

unshift removes a value from the array while push adds onto an array. This code gradually empties the array and that is why unshift is used.