createParamsConstructor.ts

import { Chunk, ParamType } from './pathParser'
import { last } from 'ramda'

const evalCache = Object.create(null)
const cachedEval = (fn: string): (path: string[]) => {} => {
  if (!evalCache[fn]) {
    /* see MDN https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval
    *  from the line "If you use the eval function indirectly..."
    **/
    const globalEval = eval

    evalCache[fn] = globalEval(`(function magicCreateParams (path) {return {${fn}}})`)
  }

  return evalCache[fn]
}

function emptyFn () {
  return {}
}

export const createParamsConstructor = (pathChunks: Chunk[]) => {
  const pathKey = 'path'
  const keys = pathChunks
    .map(({ chunk, type }, index) => type === ParamType.param ? { chunk, index } : null)
    .filter(val => val !== null)
    .map(({ chunk, index }) => `${chunk}: ${pathKey}[${index}]`)
    .join(',')
  if (keys.length === 0) {
    return emptyFn
  }
  if (last(pathChunks).type === ParamType.wildcard) {
    const tail = pathChunks.length - 1
    const restKey = last(pathChunks).chunk || '_'
    return cachedEval(`${keys}, ${restKey}: path.slice(0, ${tail}).join('/')`)
  } else {
    return cachedEval(`${keys}`)
  }
}