Time complexity of the includes method in JavaScript

Spec describes this function as linear search. Array.prototype.includes

1. Let O be ? ToObject(this value).

2. Let len be ? ToLength(? Get(O, “length”)).

3. If len is 0, return false.
4. Let n be ? ToInteger(fromIndex). (If fromIndex is undefined, this step produces the value 0.)
5. If n ≥ 0, then Let k be n.
6. Else n < 0, Let k be len + n. If k < 0, let k be 0.
7. Repeat, while k < len … Increase k by 1.

Which is quite a reasonable choice in general case (list is not sorted, list is not uniform, you do not maintain additional datastructures along with the list itself).