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 HTML5 tag is indicated to be used as container for menu with navigation links in Web site?
<section> <nav> <article>
 <li><a href="http://coursesweb.net/css/" title="CSS Course">CSS Course</a></li>
 <li><a href="http://www.marplo.net/jocuri/" title="Flash Games">Flash Games</a></li>
Which CSS property shifts an item horizontally to the left or right of where it was?
text-align clear float
.some_class {
  width: 30%;
  float: left;
Click on the Math object method which returns x, rounded downwards to the nearest integer.
Math.ceil(x) Math.abs(x) Math.floor(x)
var num = 12.34567;
num = Math.floor(num);
alert(num);       // 12
Indicate the PHP function which returns the number of characters in string.
mb_strlen() count() stristr()
$str = "string with utf-8 chars åèö";
$nrchr = mb_strlen($str);
echo $nrchr;        // 30
Watch Property

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