AngularJs Constant value gets changed
<p>I have a module named "app" in which I have defined a constant "PI", later in my controller when i am trying to change the value of constant it gets changed.</p>

<p>I am not able to Understand the purpose of <strong>constant</strong> in AngularJS.</p>

<pre><code>var app = angular.module('app', []);
app.constant('PI', 3.14159265359);

app.controller('appCtrl', function(PI) {
    var radius = 4;
    PI=2;
    // calculate area of the circle
    console.log(PI);
    var area = PI * radius * radius; 
});
</code></pre>