Typescript Spec Lazy Vars

lazy.ts

import _ from 'lodash'

interface LazyVarEntry<T = any> {
  gen   :  () => T,
  val?  :  T
}

interface LazyCache {
  [key: string]: LazyVarEntry[]
}

let cache : LazyCache = {}

export default function lazy(name : string, gen : () => any) {
  before(() => {
    cache[name] = cache[name] || []
    cache[name].push({
      gen: gen,
      val: (void 0)
    })
  })

  after(() => {
    cache[name].pop();
  })

  afterEach(() => {
    _.each(cache, (items) => {
      _.each(items, entry => entry.val = (void 0))
    });
  });
}

lazy.get = <T = any>(name : string) : T => {
  const entry = _.last(cache[name] || []);

  if (!entry) {
    throw new Error(`
      Attempting to lazy.get("${name}") without having it previously defined.
      Expected :
        lazy("${name}", () => value)
    `)
  }

  if (!entry.val) {
    entry.val = entry.gen();
  }
  return entry.val;
}