This is a polyfill for the Proxy object, part of ES6. See the MDN docs or Introducing ES2015 Proxies for more information on Proxy itself. Unlike other polyfills, this does not require Object.observe, which is deprecated. The polyfill supports just a limited number of proxy 'traps'. It also works by calling seal on the object passed to Proxy. This means that the properties you want to proxy must be known at creation time.