Vue computed does not work [duplicate]

Question

I use Vue Devtools.For example when i change numbers this way on console wm.numbers[0]=6 and i refresh Vue Devtools and look at, numbers has changed but evenNumbers has not changed.What is the problem?

Javascript Code:

var vm = new Vue({
  el:'#app',
  data: {
    numbers: [ 1, 2, 3, 4, 5 ]
  },
  computed: {
    evenNumbers: function () {
      return this.numbers.filter(function (number) {
        return number % 2 === 0
      })
    }
  }
})

Html Code:

<div id="app">
        <li v-for="n in evenNumbers">{{ n }}</li>
</div>

Output:

2
4

This is a dup of many already answered questions in the Vue tag about this gotcha with Vue array updating. If someone comes across a better target dup ping me and and I'll update. — Bert
– Bert, Commented Aug 16, 2018 at 18:49
Because of my bad english, i did not understand. But i understand now because of your explain. Thank you. — Murat Akbulut
– Murat Akbulut, Commented Aug 16, 2018 at 23:19

tony19 · Accepted Answer · 2022-07-15 21:45:49Z

use Vue.set or vm.$set, or replace the whole array with new one.

As Vue Guide: Reactivity In Depth describes:

Due to the limitations of modern JavaScript (and the abandonment of Object.observe), Vue cannot detect property addition or deletion. Since Vue performs the getter/setter conversion process during instance initialization, a property must be present in the data object in order for Vue to convert it and make it reactive.

Vue does not allow dynamically adding new root-level reactive properties to an already created instance. However, it’s possible to add reactive properties to a nested object using the Vue.set(object, key, value) method

There are also a few array-related caveats, which already discussed in List Rendering

var vm = new Vue({
  el:'#app',
  data: {
    numbers: [ 1, 2, 3, 4, 5 ]
  },
  computed: {
    evenNumbers: function () {
      return this.numbers.filter(function (number) {
        return number % 2 === 0
      })
    }
  },
  methods: {
    changeData1: function () {
      this.$set(this.numbers, 0, 6)
    },
    changeData2: function () {
      this.numbers = [ 16, 2, 3, 4, 5 ]
    }
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.16/vue.js"></script>

<div id="app">
<button @click="changeData1()">Update element with Vue.set or vm.$set</button>
<button @click="changeData2()">replace the whole array</button>
        <li v-for="n in evenNumbers">{{ n }}</li>
</div>

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