Extract of React's `shallowCompare`

react-shallow-compare.js

/**
 * Extracted React shallowEqual, because it's not exported.
 *
 * @see https://github.com/facebook/react/issues/16919
 * @see https://github.com/facebook/react/blob/master/packages/shared/shallowEqual.js
 */

const hasOwnProperty = Object.prototype.hasOwnProperty

/**
 * inlined Object.is polyfill to avoid requiring consumers ship their own
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is
 */
const is = (x, y) => (
  (x === y && (x !== 0 || 1 / x === 1 / y)) || (x !== x && y !== y) // eslint-disable-line no-self-compare
)

/**
 * Performs equality by iterating through keys on an object and returning false
 * when any key has values which are not strictly equal between the arguments.
 * Returns true when the values of all keys are strictly equal.
 */
const shallowEqual = (objA, objB) => {
  if (is(objA, objB)) return true

  if (
    typeof objA !== 'object'
    || objA === null
    || typeof objB !== 'object'
    || objB === null
  ) {
    return false
  }

  const keysA = Object.keys(objA)
  const keysB = Object.keys(objB)

  if (keysA.length !== keysB.length) return false

  // Test for A's keys different from B.
  for (let i = 0; i < keysA.length; i++) {
    if (
      !hasOwnProperty.call(objB, keysA[i])
      || !is(objA[keysA[i]], objB[keysA[i]])
    ) {
      return false
    }
  }

  return true
}

export default shallowEqual