[fergie]

You are essentially restating my question: what _are_ the possibilities with proxies? (What can you do with proxies that is much more messy/difficult/verbose to do otherwise?)

[foob]

One possibility is adding syntactic sugar to objects. I posted elsewhere in this thread about how we used them in Remote Browser, but here's a simple example of adding support for negative indexing to an array. This basically creates a shorthand for array[-n] === array[array.length - n].

    class MyArray extends Array {
      constructor(...args) {
        super(...args);
        return new Proxy(this, {
          get: (target, name) => {
            if (name && name.match && name.match(/^-\d+$/)) {
              return Reflect.get(target, this.length + parseInt(name, 10));
            }
            return Reflect.get(target, name);
          },
          set: (target, name, value) => {
            if (name && name.match && name.match(/^-\d+$/)) {
              return Reflect.set(target, this.length + parseInt(name, 10), value);
            }
            return Reflect.set(target, name, value);
          },
        });
      }
    }

You could accomplish the same thing by creating explicit setters and getters, but proxies allow you to directly intercept and modify the behavior of property access, calling an object like a function, etc. This sort of flexibility allows you to create APIs that wouldn't be possible otherwise.

[sgslo]

I've used it to extend the functionality of a library without having to monkeypatch or subclass a bunch of classes. See here: https://github.com/StephenGrider/AdvancedNodeComplete/blob/m...

The CustomPage class 'extends' through the use of a proxy a Page and Browser class, while adding its own methods on top. The proxy mediates access to the Custompage instance, and the Page and Browser. When calling a method on CustomPage, it first looks to see if the method exists on the CustomPage instance, then the Browser instance, then the Page.

I had originally wanted to subclass Page, which is implemented by the Puppeteer library, but the Browser class had a dozen direct references to the Page class. I would have had to sublcass the Browser as well and override all those methods to instead reference my CustomPage. The proxy seemed like an appropriate solution.

[bzbarsky]

You can log all property accesses on an object, e.g. for testing purposes. For example, see https://searchfox.org/mozilla-central/rev/da499aac682d0bbda5... (or at https://github.com/w3c/web-platform-tests/blob/a47dd526b9033... depending on which of these stays up more permanently).