Watcher doesn't detect modifications in array values

Question

Watcher doesn't detect modifications in array values

I am working with a basic array structure that looks like this:

dates[2]
  0:"2018-01-09T15:00:00.000Z"
  1:"2018-01-10T14:00:00.000Z"

My watcher is configured as follows:

data() {
    return {        
        dates: [],
    }
}

watch: {
    // this function will run whenever dates changes
    dates: {
        handler: function(before, after) {
            console.log('dates has been changed');
        },
        deep: true
    }
},

Currently, the watcher only detects when an element is added or removed from the array, not when the value (time) changes.

Is there a way to achieve the desired behavior?

javascript vue.js

Answer 1

Answer №1

Because of the way JavaScript works, Vue has difficulty detecting certain changes to arrays:

If you directly replace an item using its index, such as vm.items[indexOfItem] = newValue

If you change the length of the array, for example, vm.items.length = newLength

To address the first issue, the following two methods achieve the same result as vm.items[indexOfItem] = newValue, while also triggering state updates in the reactivity system:

// Using Vue.set
Vue.set(example1.items, indexOfItem, newValue)

// Using Array.prototype.splice
example1.items.splice(indexOfItem, 1, newValue)

To handle the second issue, you can use splice:

example1.items.splice(newLength)

Source: https://v2.vuejs.org/v2/guide/list.html#Caveats

Answer 2

Because of the way JavaScript works, Vue has difficulty detecting certain changes to arrays:

If you directly replace an item using its index, such as vm.items[indexOfItem] = newValue

If you change the length of the array, for example, vm.items.length = newLength

To address the first issue, the following two methods achieve the same result as vm.items[indexOfItem] = newValue, while also triggering state updates in the reactivity system:

// Using Vue.set
Vue.set(example1.items, indexOfItem, newValue)

// Using Array.prototype.splice
example1.items.splice(indexOfItem, 1, newValue)

To handle the second issue, you can use splice:

example1.items.splice(newLength)

Source: https://v2.vuejs.org/v2/guide/list.html#Caveats

Watcher doesn't detect modifications in array values

Answer №1

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