AngularJS: Constants vs Values

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.config(function(PI){
    var radius = 4;
    //PI can be injected here in the config block
    var perimeter = 2 * PI * radius;
});

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!';
    });
});