Simple JS create selector

createSelector.mjs

const UNINITIALIZED = {}

export function createSelector() {
  const nInputSelectors = arguments.length - 1
  if (nInputSelectors <= 0) {
    throw new Error('invalid number of inputs')
  }

  const transform = arguments[nInputSelectors]

  if (nInputSelectors === 1) {
    const inputSelector = arguments[0]
    let prevInput = UNINITIALIZED
    let prevResult
    return function selector() {
      const input = inputSelector.apply(this, arguments)
      if (input !== prevInput) {
        prevResult = transform.call(this, input)
        prevInput = input
      }
      return prevResult
    }
  }

  const inputSelectors = Array.prototype.slice(arguments, 0, nInputSelectors)
  const prevInputs = Array.from({ length: nInputSelectors }, () => UNINITIALIZED)
  let prevResult
  return function selector() {
    let inputsChanged = false
    for (let i = 0; i < nInputSelectors; ++i) {
      const input = inputSelectors[i].apply(this, arguments)
      if (input !== prevInputs[i]) {
        inputsChanged = true
        prevInputs[i] = input
      }
    }
    if (inputsChanged) {
      prevResult = transform.apply(this, prevInputs)
    }
    return prevResult
  }
}