const vm = new Vue({
 data: {foo: 1} // 'vm.foo' (在内部，同 'this.foo') 是响应的
})

const vm = new Vue({
 data: {foo: 1}
})

vm.$watch('foo', function (newValue, oldValue) {
 console.log(newValue, oldValue) // 输出 2 1
 console.log(this.foo) // 输出 2
})

vm.foo = 2

const vm = new Vue({
 data: {foo: 1}
})

vm.$watch('foo', function (newValue, oldValue) {
 console.log('inner:', newValue) // 后输出 "inner" 2
})

vm.foo = 2
console.log('outer:', vm.foo) // 先输出 "outer" 2

const vm = new Vue({
 data: {foo: 1}
})

vm.$watch('foo', function (newValue, oldValue) {
 console.log('inner:', newValue) // 后只输出一次 "inner" 5
})

vm.foo = 2
vm.foo = 3
vm.foo = 4
console.log('outer:', vm.foo) // 先输出 "outer" 4
vm.foo = 5

const vm = new Vue({
 data: {
 width: 0,
 height: 0,
 },
 computed: {
 area () {
  let output = ''
  if (this.width > 0 && this.height > 0) {
  const area = this.width * this.height
  output = area.toFixed(2) + 'm²'
  }
  return output
 }
 }
})

vm.width = 2.34
vm.height = 5.67
console.log(vm.area) // 输出 "13.27m²"

const vm = new Vue({
 data: {
 width: 0,
 height: 0,
 },
 computed: {
 area () {
  let output = ''
  const {width, height} = this
  if (width > 0 && height > 0) {
  const area = width * height
  output = area.toFixed(2) + 'm²'
  }
  return output
 }
 }
})

vm.width = 2.34
vm.height = 5.67
console.log(vm.area) // 输出 "13.27m²"

npm install --save ob.js

const target = {a: 1}
ob(target, 'a', function (newValue, oldValue) {
 console.log(newValue, oldValue) // 3 1
})
target.a = 3

const target = {a: 1}
ob.compute(target, 'b', function () {
 return this.a * 2
})
target.a = 10
console.log(target.b) // 20

const options = {
 data: {
 PI: Math.PI,
 radius: 1,
 },
 computed: {
 'area': function () {
  return this.PI * this.square(this.radius)
 },
 },
 watchers: {
 'area': function (newValue, oldValue) {
  console.log(newValue) // 28.274333882308138
 },
 },
 methods: {
 square (num) {
  return num * num
 },
 },
}
const target = ob.react(options)
target.radius = 3