Vue.js provides us with a way to programmatically react to value changes in other properties. These are known as watcher functions, and they are defined through the watch option.

Each watcher is defined as a property name – function mapping, where the property name is the name of the property to watch and the function is called whenever the property value changes.

Here's an example:

var watcherExample = new Vue({ 
  el: '#watcher-example', 
  data: { 
    something: '', 
    lastUpdated: '', 
  }, 
  watch: { 
    // whenever 'something' changes the function below will get
     invoked something: function (newValue, oldValue) { 
      console.log(`Old value: ${oldValue}`); 
      console.log(`New value: ${newValue}`); 
      this.lastUpdated = Date.now(); 
    } 
  }, 
}) 

In this example, whenever the value of the something property changes, the associated watch function will be called.

We'll look at filters next.