Why does resolving a promise with a proxy and then calling 'then' access the Proxy's getter?

Here's a minimal example:

const proxy = new Proxy({}, {
  get (target, name) {
    console.error('Getter ' + name + ' should not be called')
  }
});

const promise = Promise.resolve(proxy);

promise.then((proxy) => proxy);

In my mind, the call to then is accessing the property on the Promise, and so the getter on Proxy should never be accessed in the above example. Is this expected behaviour, or a bug? And, if it is expected, why?

promise.then on its own is just fine, and is not invoking the proxy. For example, add in the following line of code, and you won't see an additional use of the proxy:

const temp = promise.then;

Instead, this comes down to the specification for promises. When you call resolve(proxy), part of the promise resolution procedure includes checking the value of proxy.then. Then based on whether proxy.then is a function or not, it affects how the resolution process continues. Since proxy.then is being accessed, you're seeing the log statements.

See section 2.3 of the Promise A+ spec, in particular subsection 2.3.3.1