Metaprogramming with Proxy

In an earlier example, in the Injecting Multiple Properties lesson, we introduced a few missing properties like first and last into arrays. This is member injection; we knew what to introduce or inject at code writing time. While that’s fun and useful, member synthesis ups the challenge and the resulting benefits by a few notches:

• With synthesis, we can bring onboard new members into a class based on runtime context.
• We can also alter the behavior of existing methods or properties dynamically based on the current state at runtime.

To achieve this, we need the help of the Proxy class. We’ll first explore this newly added class in JavaScript and then employ it for method synthesis.

The Proxy class

Definition: An instance of the Proxy class stands in for another object or a function, also known as target, and can intercept or trap calls to fields, methods, and properties on its target.

To create a proxy, provide two things:

1. A target—the proxy stands in for this.
2. A handler—this traps and intercepts calls on the target.

Use the handler to trap any operation that may be performed on an object or a function. By default, if you don’t trap a call, it defaults to a method on Reflect so that the calls are forwarded to the target.