shallowEqual always return false if add properties to Object.prototype

[LiuuY]

// copy from src/utils/shallowEqual.js
const hasOwn = Object.prototype.hasOwnProperty

function shallowEqual(a, b) {
  if (a === b) return true

  let countA = 0
  let countB = 0
  
  for (let key in a) {
    if (hasOwn.call(a, key) && a[key] !== b[key]) return false
    countA++  // should check hasOwn.call(a, key) === true and then countA++ ?
  }

  for (let key in b) {
    if (hasOwn.call(b, key)) countB++
  }

  return countA === countB
}

// my example
Object.prototype.bar = 'bar'

let a = {x: '1'}
let b = {x: '1'}

shallowEqual(a, b) // return false

it's a misunderstanding or there are some reasons for this?

[timdorr]

Well first off, you shouldn't be adding to primitives, as that affects the entire system and can have unexpected side effects.

We can probably swap out for fbjs's version instead, since it covers these edge cases and might be a bit faster too.