Difference between constant and value

A constant can be injected anywhere.

A constant can not be intercepted by a decorator, that means that the value of a constant should never be changed.

var app = angular.module('app', []);

app.constant('PI', 3.14159265359);

app.controller('appCtrl', function(PI) {

    var radius = 4;

    // calculate area of the circle

    var area = PI * radius * radius;

});

Value differs from constant in that value can not be injected into configurations, but it can be intercepted by decorators.

var app = angular.module('app', []);

app.value('greeting', 'Hello');

app.config(function ($provide) {

    $provide.decorator('greeting', function ($delegate) {

        return $delegate + ' World!';

    });

});