A watcher allows you to "watch" a property of the component data, and run a function when that property value changes.
In the watch object add a method with the same name as the property data you want to "watch".

var vm = new Vue({
 el: 'css_selector',
 data: {
 prop_name: 'value'
 watch: {
 prop_name: function(newVal, oldVal{
 //code and value to return

The function assigned to watch.prop_name can optionally accept 2 parameters. The first is the new property value (newVal). The second is the old property value.

Here is a practical example with the watch property in vue.js. Add the diameter of a circle into an input field, then it is displayed the perimeter and area:
<div id='demo'>
 Diameter circle: <input type='number' value='0' v-model='diameter'/><br>
 Perimeter: {{perimeter}}<br>
 Area: {{area}}

var vm = new Vue({
 data: {
 diameter: 0,
 watch: {
 diameter: function(val){
 this.perimeter = 3.14*val;
 this.area = 3.14*(val*val)/4;

Daily Test with Code Example

Which type of <input> creates a date input control, such as a pop-up calendar?
type="text" type="date" type="button"
<input type="date" name="set_date" value="2012-10-15" />
Which CSS property adds shadow effects to the text of an element?
font-style color text-shadow
h2 {
  text-shadow: 2px 3px 3px #a0a1fe;
Click on the function that adds new elements to the end of an array.
pop() shift() push()
var pags = ["lessons", "courses"];
pags.push("download", "tutorials");
alert(pags[2]);            // download
Which function sorts an array by key, in ascending order, maintaining key to data correlations?
asort() ksort() sort()
$lang =[10=>"PHP", 20=>"JavaScript", "site"=>"coursesweb.net");
var_export($lang);     // array ("site"=>"coursesweb.net", 10=>"PHP", 20=>"JavaScript")
Watch Property

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