How does this conditional statement work?-CodePudding

This function finds the middle value of an array. But the conditional statement makes no sense to me, since arr.length === 0 is never true but the function still works for both even and odd numbers. I think the condition should be arr.length % 2 !== 0 and it works too, but even then I don't see how the correct value is returned, since for an array of 1-13, the function would return the Math.floor of 13/2 = 6.5 which is 6, but it returns 7. And for an array 1-12 it would return the Math.ceil of (12-1)/2 = 5.5 which is also 6, but again it returns 7, which is what it's supposed to do but I don't see how the conditional statements produce this result.

let x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
// if even, the number closer to end is returned

function findMiddle(arr) {
   return arr.length === 0 ? arr[Math.floor(arr.length / 2)]
   : arr[Math.ceil((arr.length - 1) / 2)];
}

console.log(findMiddle(x))

CodePudding user response：

If arr has no elements, this function returns arr[0] which does not exist, so it throws an error. (edit: this would happen in any other language, but javascript being javascript actually returns undefined)

For an even number of elements, it returns the second of the two middle-elements.

For an uneven number of elements it returns the middle one.

In your example, it returns the element at index 6. This element is has the value 7.

CodePudding user response：

Since the condition is always false, this function will always return the first value which is arr[Math.ceil((arr.length - 1) / 2)] which is always correct I assume.

For the first example (an array from 1 - 13): arr.length is 13, arr.length / 2 will be 6.5, then Math.ceil(arr.length / 2) will be 7 and finally arr[Math.ceil(arr.length / 2)] will result in arr[7] which is 8
For the second example (an array from 1 - 12): arr.length is 12, arr.length / 2 will be 6, then Math.ceil(arr.length / 2) will be 6 and finally arr[Math.ceil(arr.length / 2)] will result in arr[6] which is 7.

Conclusion: The condition is not doing anything there. It's completely useless if you always provide it with non-empty arrays. Your function, in that case, will be simply:

function findMiddle(arr) {
   return arr[Math.ceil(arr.length / 2)]
}

And it will give the same results.