Allow providing the this context to getValue and getKeyValue

[matthewp]

Reflect.get takes a 3rd argument, the this value that will be passed into getters. This is useful. Would be nice to have in the can-reflect APIs as well.

[justinbmeyer]

What is this used for?

[matthewp]

Changing the context in getters.

[justinbmeyer]

Yeah, but why? I'm not arguing against changing it, I'm just wondering what is the use case. I understand why this is important for functions, but normally property access is so "bound/connected" to the object, in having trouble seeing where this is useful. Perhaps being able to "steal" something like Array's length.

…

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[matthewp]

I don't have a specific use-case in can-reflect at the moment. In regular old js, when overriding a Proxy getter you would want this feature so that if calling into the target getter the Proxy object would be the value of this (so if any property access happens there it also goes through the Proxy).

Just opened this for discussion, no reason to add until a clear use-case is present.

[matthewp]

Here's some pseudocode where this would be useful with Proxys. Might help think of a where this could also be important with can-reflect:

var target = {
  foo: "bar"
};
Object.defineProperty(target, "baz", {
  get: function(){
    return this.foo;
  }
});

var p = new Proxy(target, {
  get: function(target, prop){
    // Count all observations
    Observation.add(target, prop);

    // This means that the `foo` accessor will be observed
    return Reflect.get(target, prop, p);
  }
});

// This should count 2 observations
console.log(p.baz);