A watcher is a special Vue.js feature that allows you to spy on one property of the component state, and run a function when that property value changes.
Here’s an example. We have a component that shows a name, and allows you to change it by clicking a button:
<template>
<p>My name is {{name}}</p>
<button @click="changeName()">Change my name!</button>
</template>
<script>
export default {
data() {
return {
name: 'Flavio'
}
},
methods: {
changeName: function() {
this.name = 'Flavius'
}
}
}
</script>When the name changes we want to do something, like printing a console log.
We can do so by adding to the watch object a property named as the data property we want to watch over:
<script>
export default {
data() {
return {
name: 'Flavio'
}
},
methods: {
changeName: function() {
this.name = 'Flavius'
}
},
watch: {
name: function() {
console.log(this.name)
}
}
}
</script>The function assigned to watch.name can optionally accept 2 parameters. The first is the new property value. The second is the old property value:
<script>
export default {
/* ... */
watch: {
name: function(newValue, oldValue) {
console.log(newValue, oldValue)
}
}
}
</script>Watchers cannot be looked up from a template as you can with computed properties.