JavaScript Decorator Design Pattern

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An example of a decorator is security management where business objects are given additional access to privileged information depending on the privileges of the authenticated user. For example, an HR manager gets to work with an employee object that has appended (i.e. is decorated with) the employee's salary record so that salary information can be viewed.

Decorators provide flexibility to statically typed languages by allowing runtime changes as opposed to inheritance which takes place at compile time. JavaScript, however, is a dynamic language and the ability to extend an object at runtime is baked into the language itself.

For this reason, the Decorator pattern is less relevant to JavaScript developers. In JavaScript the Extend and Mixin patterns subsume the Decorator pattern. You can learn more about these and other Modern JavaScript patterns in our Dofactory JS.

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The objects participating in this pattern are:

• Client -- In example code: the run() function
  • maintains a reference to the decorated Component
• Component -- In example code: User
  • object to which additional functionality is added
• Decorator -- In example code: DecoratedUser
  • 'wraps around' Component by maintaining a reference to it
  • defines an interface that conforms to Component's interface
  • implements the additional functionality (addedMembers in diagram)

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In the example code a User object is decorated (enhanced) by a DecoratedUser object. It extends the User with several address-based properties. The original interface must stay the same, which explains why user.name is assigned to this.name. Also, the say method of DecoratedUser hides the say method of User.

JavaScript itself is far more effective in dynamically extending objects with additional data and behavior. You can learn more about extending objects at runtime and other techniques in our Dofactory JS.

var User = function (name) {
    this.name = name;

    this.say = function () {
        console.log("User: " + this.name);
    };
}

var DecoratedUser = function (user, street, city) {
    this.user = user;
    this.name = user.name;  // ensures interface stays the same
    this.street = street;
    this.city = city;

    this.say = function () {
        console.log("Decorated User: " + this.name + ", " +
            this.street + ", " + this.city);
    };
}

function run() {

    var user = new User("Kelly");
    user.say();

    var decorated = new DecoratedUser(user, "Broadway", "New York");
    decorated.say();
}

Run

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