javascript-includes.md

Question

import { Eq } from 'fp-ts/Eq'

type Point = {
  readonly x: number
  readonly y: number
}

const EqPoint: Eq<Point> = {
  equals: (first, second) => first.x === second.x && first.y === second.y
}

const points: ReadonlyArray<Point> = [
  { x: 0, y: 0 },
  { x: 1, y: 1 },
  { x: 2, y: 2 }
]

const search: Point = { x: 1, y: 1 }

console.log(points.includes(search)) // => false :(
console.log(pipe(points, elem(EqPoint)(search))) // => true :)

Why does the includes method returns false?

Answer

The includes method compares by value in case of primitive values, and by reference in other cases.

As explained here, includes() uses the sameValueZero algorithm to determine whether the given element is found.

The sameValueZero algorithm is very close to the one used by === (see details here) and objects are compared through their references instead of their values:

console.log({ foo: 'bar' } === { foo: 'bar' }) // => false

const foo = { foo: 'bar' }
console.log(foo === foo) // => true