const TOKEN_REG = /\{\{(\w+)\}\}/g

​

class Vue {

  constructor (config) {

    this.config = config

    this.template = config.el.innerHTML

    this._init()

  }

  _init () {

    this.render()

  }

  render () {

    let data = this.config.data

    this.config.el.innerHTML = this.template.replace(TOKEN_REG, function (_, val) {

      return data[val.trim()]

    })

  }

}

​

let vm = new Vue({

  el: app,

  data: {

    message: 'Hello Vue.js!'

  }

})

​

changeBtn.addEventListener('click', handleChange)

function handleChange () {

  vm.config.data.message = 'Hello world!'

  vm.render()

}