Easy implementation of Promise Finally using native JS

finally.js

Promise.prototype['finally'] = function finallyPolyfill(callback) {
    var constructor = this.constructor;

    return this.then(function(value) {
        return constructor.resolve(callback()).then(function() {
            return value;
        });
    }, function(reason) {
        return constructor.resolve(callback()).then(function() {
            throw reason;
        });
    });
};