你能为我解释一下这个 JavaScript function 吗？

Question

 function countup(n) {
  if (n < 1) {
    return [];
  } else {
    const countArray = countup(n - 1);
    countArray.push(n);
    return countArray;
  }
}
console.log(countup(5)); // [ 1, 2, 3, 4, 5 ]

Could you guys please explain to me how this function works and when the variable "countArray" would push the elements to the array.

Answer 1

It is a recursive function that puts all numbers from 1 to n into an array. Lets look at the example with n = 5 :

you enter the function and check if n is smaller than 1 , as this is not the case, you jump into the else statement, where the array countArray is initialized with the value of countup(n - 1) , therefore you start from the beginning of the function, but with n = 4 .

This happens multiple times, until n is 0 , as in that case, the first if statement is true. By this exit condition, the function returns an empty array, which is assigned to the variable countArray in the function call in the previous 'layer' of recursion. After that, the variable n which equals 1 on that 'layer' is pushed into the array. These last steps also happen multiple times, as countArray is returned to the next call of the recursive function, in the same way as the empty array was returned.

After all recursive function calls were executed the resulting array [1,2,3,4,5] is returned

Answer 2

This function will check first if the variable n < 1

If it is bigger it will run itself with n-1

If n is 0 , the function will return an empty Array and will go one "layer" back to the last function countup(1) It will now add n (1) to the countArray and return this Array to the "last" function Now the function countup(2) has recieved the number 1 in its countArray and it will return this. [1, 2]

It will repeate this process until the 'last' / 'first' function ( countup(5) ) is reached and it will return the countArray with all the numbers [1, 2, 3, 4, 5]

Answer 3

To understand recursions, sometimes you need to follow the execution of the code manually. Now the following line causes the recursive call and subtracts one from the number and then passes it to the recursive call.

const countArray = countup(n - 1);

Now imagine, if you passed 5 to the initial function call like so:

countup(5);

As 5 is not less than 1, the else block will be executed. The first line in the else block is the recursive call. So when your program encounters the recursive call, it stops execution of the current function until that recursive function call is concluded/returned. So the first recursive call will be:

const countArray = countup(4);

consequently, the same process will be repeated until n equals 0 . As 0 is smaller than 1, the if condition will evaluate to true . and it will return an empty array .

After the recursive call, it pushes the number to the array returned by recursive call. so the array at that point will contain [0] and then [0, 1] and it will keep adding numbers till the all recursive calls have been computed and then it will execute the rest of the intial function call.

Often times, recursion can be replaced by loops but thats not the case always. As a programmer one should be aware of this important topic.