1089128 - DOM Promise are too eager to call `then`

Status: NEW

(Core :: DOM: Core & HTML, defect, P5)

Description

[David Teller [:Yoric] - still alive but not very active]

Here I am, attempting to sanitize code by adding the following proxy:

var proxy = new Proxy({}, {
  get: function(target, name) {
    if (name in target) {
      return target[name];
    }
    throw new Error("No such property: " + name);
  }
});

Unfortunately, `proxy` cannot be used to resolve a promise:

var p = new Promise(resolve => resolve(proxy));
p.then(() => console.log("Success"),
       ex => console.error(ex));
// Displays: "No such property: then"

I expect that, since `then` is not a key of `proxy`, we should never get `proxy.then`.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Per spec at http://people.mozilla.org/~jorendorff/es6-draft.html#sec-promise-resolve-functions there is no check for "then" being a key.  As long as the resolution value is not the promise itself and Type(resolution) is Object, we get to step 8, which does:

  8.  Let then be Get(resolution, "then").

In general, all sorts of algorithms in the spec use Get, not a combination of HasProperty and Get, and this proxy would fail for those.  Another example is passing this proxy to Array.from if the target has a "length" property but is not iterable.  Since GetIterator simply does a Get on @@iterator, you'll end up throwing...

That said, it may be worth a spec issue here since resolving promises with filtering proxies seems like a much more sane use case than passing such proxies to Array.from.

Comment 2

[Firefox Bug Husbandry Bot]

https://bugzilla.mozilla.org/show_bug.cgi?id=1472046

Move all DOM bugs that haven’t been updated in more than 3 years and has no one currently assigned to P5.

If you have questions, please contact :mdaly.