Discussion on: Please don't "overchain" array methods

[Mihail Malo]

I wish JS runtimes did better inlining and monomorphization.
Then you could have your own utilities like

export const map => (arr, fn) {
  const len = arr.length
  const out = new Array(len)
  for (let i = 0; i < len; i++) {
    out[i] = fn(arr[i])
  }
  return out
}

Which beats out what the spec method does:

export function map(fn, thisArg) {
  const O = Object(this)
  const len = O.length
  if (!(fn instanceof Function)) throw new TypeError(`${fn} is not a function`)
  const A = new Array(len)
  for (let k = 0; k < len; k++) {
    const Pk = k.toString()
    const kPresent = O.hasOwnProperty(Pk)
    if (kPresent) {
      const kValue = O[i]
      const mappedValue = fn.call(thisArg, kValue, i, O)
      Object.defineProperty(A, Pk, { value: mappedValue, writable: true, enumerable: true, configurable: true })
    }
  }
  return A
}

But most of that is lost unless you actually write the upper loop inline, because unlike the built in methods it gets deoptimized if you call the utility with many different fn functions. Especially when some of them throw.