// *** before: ***
var arr = [1, 2, NaN];
console.log(arr.indexOf(2) !== -1); // true
console.log(arr.indexOf(NaN) !== -1); // false - NaN !== NaN

// *** in version ES2016: ***
const arr = [1, 2, NaN];
console.log(arr.includes(2)); // true
console.log(arr.includes(NaN)); // true - includes использует SameValueZero