This is not node specific but ECMA (JS) specific.

in Operator

Checks the existence of key in the collection (similar to hasOwnPropertybut also check inherited keys in prototype chain)

includes method of Array (introduced in ES6)

Checks the existence of value in the collection

Array.includes() checks for the existence of a certain value in an array, while the in operator checks for the existence of a key in an object (or an index in the case of arrays like you're describing).

console.log('a' in ['a', 'b']); // false, no such key
console.log(0 in ['a', 'b']); // true, 0 is a key that exists
console.log(1 in ['a', 'b']); // true, 1 is a key that exists
console.log(2 in ['a', 'b']); // false, 2 is a key that doesn't exists