Why not always use the index as the key in a vue.js for loop?

Question

I have used vue.js for a couple of projects and I have been using the index as the key in the for loops

Answer 1

Because arrays are mutable. The index of any given item can and will change if items are added to or removed from the array.

You want your key to be a unique value identifying only your unique component. A primary key that you create is always better than using an index.

Here is an example.

console.clear()

Vue.component("item", {
  props: ["value"],
  data() {
    return {
      internalValue: this.value
    }
  },
  template: `
Internal: {{internalValue}} Prop: {{value}}
`
})


new Vue({
  el: "#app",
  data: {
    items: [1, 2, 3, 4, 5]
  },
  methods: {
    addValue() {
      this.items.splice(this.items.length / 2, 0, this.items.length + 1)
    }
  }
})

  {{items}}
  

    
  
  AddValue
  

    
  

Note that when addValue is clicked, the list on top represents the new numbers in the array where the truly are in the array; in the middle. In the second list below the button, the values do not represent the actual location in the array and the internal and property values do not agree.