params.js

const stack = []

// exported for tests
export let target = null
export const _setTarget = x => { target = x }

export function pushParams () {
  if (target !== null) {
    stack.push(target)
  }
  target = {}
}

export function popParams () {
  const lastTarget = target
  const newTarget = target = stack.pop() || null
  if (newTarget) {
    if (!Array.isArray(newTarget.$sub)) {
      newTarget.$sub = []
    }
    newTarget.$sub.push(lastTarget)
  }
  return lastTarget
}

function addParams (params) {
  if (typeof params === 'object' && !Array.isArray(params)) {
    target = {...target, ...params}
  } else {
    throw new Error('params must be an object')
  }
}

function withParamsDirect (params, validator) {
  return withParamsClosure(add => {
    return function (...args) {
      add(params)
      return validator.apply(this, args)
    }
  })
}

function withParamsClosure (closure) {
  const validator = closure(addParams)
  return function (...args) {
    pushParams()
    try {
      return validator.apply(this, args)
    } finally {
      popParams()
    }
  }
}

export function withParams (paramsOrClosure, maybeValidator) {
  if (typeof paramsOrClosure === 'object' && maybeValidator !== undefined) {
    return withParamsDirect(paramsOrClosure, maybeValidator)
  }
  return withParamsClosure(paramsOrClosure)
}