cls-ah.js

// async_hooks-based continuation-local storage (simplified)

const asyncHooks = require('async_hooks')

const contexts = {}
let current = {}
asyncHooks.createHook({
  init: uid => {
    contexts[uid] = current
  },
  before: uid => {
    if (contexts[uid]) {
      current = contexts[uid]
    }
  },
  destroy: uid => {
    delete contexts[uid]
  }
}).enable()

module.exports = {
  bindInCurrentContext: fn => { // akin to bind
    const boundContext = current
    return (...args) => {
      const oldContext = current
      current = boundContext
      const result = fn(...args)
      current = oldContext
      return result
    }
  },
  bindInNewContext: fn => { // akin to runAndReturn (but lazy)
    return (...args) => {
      const oldContext = current
      current = {}
      const result = fn(...args)
      current = oldContext
      return result
    }
  },
  set: (k, v) => {
    current[k] = v
  },
  get: k => current[k]
}

index.js

const { bindInCurrentContext, bindInNewContext, set, get } = require('./cls-ah')

const wait = ms => ({ then: (fn) => setTimeout(fn, ms) })

bindInNewContext(async () => {
  set('key', 2)
  console.log('A', get('key'))
  await wait(0)
  console.log('B', get('key'))
})()