Array prototype is read only, properties should not be added no-extend-native

Its because esLint gets it mutation to the native protoTypes chain. You can add // eslint-disable-next-line no-extend-native in above the line it should be fine.

From EsLint Docs:

In JavaScript, you can extend any object, including builtin or “native” objects. Sometimes people change the behavior of these native objects in ways that break the assumptions made about them in other parts of the code.

For example here we are overriding a builtin method that will then affect all Objects, even other builtin.

// seems harmless
Object.prototype.extra = 55;

// loop through some userIds
var users = {
    "123": "Stan",
    "456": "David"
};

// not what you'd expect
for (var id in users) {
    console.log(id); // "123", "456", "extra"
}

In-short, Array.prototype.inState will extend the array.prototype and hence whenever you want to use an array, the instate function will be added to that array too.

So, in your case this example will be applied to array.

Array.prototype.inState = function (needle, haystack) {
  let index = this.findIndex(value => value[needle] === haystack);

  return index === -1;
};


// loop through some userIds
var users = [{"123": "Stan"},{"456": "David"}];

// not what you'd expect
for (var id in users) {
    console.log(users[id]); // "123", "456", "extra"
}

WorkAround

You can add this line to ignore warning.

/*eslint no-extend-native: ["error", { "exceptions": ["Object"] }]*/ to ignore that warning.

Ref: https://eslint.org/docs/rules/no-extend-native