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VueJs Watchers

Examples

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'
 }
 }
}


Conclusion

In this page (written and validated by ) you learned about VueJs Watchers



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