I can't understand why IndexOf returned -1

Question

I have an array of numbers containing 1-20. When I use the indexOf method to try to find the position of "2", it shows up at the 3rd position (correct).

I have a variable called day = 2. When I use the code of array.indexOf(day), it returns -1.

Why does this happen?

Answer 1

function indexofnotworking() {
  let arr = [...new Array(20).keys()];//creat arr of numbers from 0  -19
  let arr1 = arr.slice().reverse();//make a copy and reverse it
  [...Array.from(new Array(5).keys(), x => x + 5)].forEach(n => {
    console.log('arr: %s n: %s index: %s', JSON.stringify(arr), n, arr.indexOf(n));//find the indexof n in both arrays
    console.log('arr1: %s n: %s index: %s', JSON.stringify(arr1), n, arr1.indexOf(n));/display results
  })
}

11:40:22 AM Info    arr: [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] n: 5 index: 5
11:40:22 AM Info    arr1: [19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0] n: 5 index: 14
11:40:22 AM Info    arr: [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] n: 6 index: 6
11:40:22 AM Info    arr1: [19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0] n: 6 index: 13
11:40:22 AM Info    arr: [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] n: 7 index: 7
11:40:22 AM Info    arr1: [19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0] n: 7 index: 12
11:40:22 AM Info    arr: [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] n: 8 index: 8
11:40:22 AM Info    arr1: [19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0] n: 8 index: 11
11:40:22 AM Info    arr: [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] n: 9 index: 9
11:40:22 AM Info    arr1: [19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0] n: 9 index: 10