Watchers

How it works

You can watch data property of any Vue instance. When watching a property, you trigger a method on change:

export default {
    data () {
        return {
            watched: 'Hello World'
        }
    },
    watch: {
        'watched' () {
            console.log('The watched property has changed')
        }
    }
}

You can retrieve the old value and the new one:

export default {
    data () {
        return {
            watched: 'Hello World'
        }
    },
    watch: {
        'watched' (value, oldValue) {
            console.log(oldValue) // Hello World
            console.log(value) // ByeBye World
        }
    },
    mounted () {
        this.watched = 'ByeBye World'
    }
}

If you need to watch nested properties on an object, you will need to use the deep property:

export default {
    data () {
        return {
            someObject: {
                message: 'Hello World'
            }
        }
    },
    watch: {
        'someObject': {
            deep: true,
            handler (value, oldValue) {
                console.log('Something changed in someObject')
            }
        }
    }
}

When is the data updated?

If you need to trigger the watcher before making some new changes to an object, you need to use the nextTick() method:

export default {
    data() {
        return {
            foo: 'bar',
            message: 'from data'
        }
    },
    methods: {
        action () {
            this.foo = 'changed'
            // If you juste this.message = 'from method' here, the watcher is executed after. 
            this.$nextTick(() => {
                this.message = 'from method'
            })
        }
    },
    watch: {
        foo () {
            this.message = 'from watcher'
        }
    }
}