js indexof bug

array.indexOf(searchElement[, fromIndex])
string.indexOf(searchValue[, fromIndex])

let str = "Hello World";
console.log(str.indexOf("world")); // 输出: -1

let str = "Hello World";
console.log(str.toLowerCase().indexOf("world".toLowerCase())); // 输出: 6

let arr = [1, 2, NaN, 4];
console.log(arr.indexOf(NaN)); // 输出: -1

let arr = [1, 2, NaN, 4];
console.log(arr.includes(NaN)); // 输出: true

function includesNaN(array) {
  return array.some(element => element !== element);
}
console.log(includesNaN(arr)); // 输出: true

let sparseArr = [1, , 3];
console.log(sparseArr.indexOf(3)); // 输出: 2
console.log(sparseArr.length); // 输出: 3

let sparseArr = [1, , 3];
for (let index in sparseArr) {
  console.log(index, sparseArr[index]); // 输出: "0" 1 和 "2" 3
}