What is the difference between "in operator" and "includes()" for JavaScript arrays

Question

I mean, includes is an Array prototype but in can be used with arrays too, so what is the main difference between both?

Answer 1

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

Example:

var arr = [5];

console.log('Is 5 is an item in the array: ', arr.includes(5)); // true
console.log('do we have the key 5: ', 5 in arr); // false
console.log('we have the key 0: ', 0 in arr); // true

Answer 2

"includes" checks whether a value exists in an array where as "in" operator checks whether a key/index exists in obj/array.

var arr = [15, 27, 39, 40, 567],
  obj = {
    num1: 3,
    num2: 34,
    num3: 89
  };;
console.log(arr.includes(27)); // returns true checks 27 as a value in the array
console.log(2 in arr);         // returns true checks 2 as index in the array
console.log("num1" in obj);    // returns true checks num1 as key in obj