computed vs watch vue

1// ...
2computed: {
3  fullName: {
4    // getter
5    get: function () {
6      return this.firstName + ' ' + this.lastName
7    },
8    // setter
9    set: function (newValue) {
10      var names = newValue.split(' ')
11      this.firstName = names[0]
12      this.lastName = names[names.length - 1]
13    }
14  }
15}
16// ...

1var vm = new Vue({
2  el: '#example',
3  data: {
4    message: 'Hello'
5  },
6  computed: {
7    // a computed getter
8    reversedMessage: function () {
9      // `this` points to the vm instance
10      return this.message.split('').reverse().join('')
11    }
12  }
13})

1vm.$watch('person.name.firstName', function(newValue, oldValue) {
2	alert('First name changed from ' + oldValue + ' to ' + newValue + '!');
3});

1Differences and Similarities
2	Let's see how they are different:
3
4        	Computed props are more declarative than watched properties
5            
6        	Computed props should be pure: return a value, synchronous, and 
7        have no side-effects
8        
9        	Computed props create new reactive properties, watched props only 
10        call functions
11        
12        	Computed props can react to changes in multiple props, whereas 
13        watched props can only watch one at a time.
14        
15        	Computed props are cached, so they only recalculate when things 
16        change. Watched props are executed every time
17        
18        	Computed props are evaluated lazily, meaning they are only 
19        executed when they are needed to be used. Watched props are executed 
20        whenever a prop changes
21
22  	But they also have some similarities
23    
24	They aren't exactly twins, but they both:
25
26          React to changes in properties
27          Can be used to compute new data
28          
29    Watch is for side effects. If you need to change state you want to use a computed prop instead.
30
31	Most of the time you'll want a computed prop, so try to use that first. 
32    If it doesn't work, or results in weird code that makes you feel like 
33    taking a shower, then you should switch to using a watched prop.