Use ES6 Proxy to attach computed properties

computed-property.js

'use strict';

const VALUE = Symbol.for('value')

function addComputedProperty(obj, name, func) {
  let _computed = func(obj)

  return new Proxy(obj, {
    get(target, key, receiver) {
      if (key === name) {
        return _computed
      }
      return Reflect.get(target, key, receiver)
    },

    set(target, key, value, receiver) {
      if (key === name) {
        throw new ReferenceError(`Cannot set computed property "${name}"`)
      }
       
      if (value === Reflect.get(target, key, receiver)) {
        return true
      }
       
      Reflect.set(target, key, value, receiver)

      _computed = func(target, key)
    }
  })
}

let q = addComputedProperty({}, VALUE, obj => {
  const keys = Object.keys(obj)

  if (keys.length === 0) {
    return ''
  }

  return '?' + keys.map(k => k + '=' + encodeURIComponent(obj[k])).join('&')
})