Currently, inspecting a proxy object executes some of its traps. This can have the collateral effect of modifying the target object or the future behavior of the proxy traps, and thus the displayed data can be completely incoherent. In the same way that getters of accessor properties are not executed during inspection, proxy traps shouldn't neither. As proposed by Jason Orendorff in bug 1273235 comment #12: > It might be better, when displaying a proxy, for devtools to refuse to play > the proxy's stupid game, and show Proxy specially as an object with two > properties, [[ProxyHandler]] and [[ProxyTarget]].

Basically, if the object is a proxy, lie and make a grip with an invented class "Proxy". Add a pair of internal properties with the target and the handler. And avoid getOwnPropertyNames, isExtensible, getOwnPropertyDescriptor, etc. Well, bug 1282257 has not landed yet, but is something like this appropriate?

Comment on attachment 8772685 [details] [diff] [review] When inspecting a proxy, show the [[ProxyHandler]] and [[ProxyTarget]] instead of executing traps Review of attachment 8772685 [details] [diff] [review]: ----------------------------------------------------------------- Looks good to me!

I have added tests.

Argh, I forgot to update a comment.

Comment on attachment 8774085 [details] [diff] [review] When inspecting a proxy, show the [[ProxyHandler]] and [[ProxyTarget]] instead of executing traps Review of attachment 8774085 [details] [diff] [review]: ----------------------------------------------------------------- Nice, looks good ::: devtools/client/debugger/test/mochitest/browser_dbg_variables-view-07.js @@ +46,5 @@ > + data.expand(); > + yield expanded; > + if (property === "<target>") { > + for(let [subprop, subdata] of data) if(subprop === "name") { > + is(subdata.value, "target", "The value of '<target>' should be the [[ProxyTarget]]"); Isn't the display value "<target>" not "target"? ::: devtools/client/shared/widgets/VariablesViewController.jsm @@ +342,5 @@ > + let deferred = defer(); > + deferred.resolve(); > + return deferred.promise; > + } > + Nit: trailing space

(In reply to James Long (:jlongster) from comment #5) > Isn't the display value "<target>" not "target"? Yes, but that is testing the value, not the pseudo-property name. I would have preferred something like `data.value === target`, but `data.value` is a grip, and even if I wrapped `target` in another grip, they would be different references. So I added `target.name = "target"`, and then I iterate the subproperties of `proxy["<target>"]`. Since `"target"` is a primitive there is no problem when comparing. It's not perfect, but in that test there isn't any other object with `.name = "target"`. And all traps of the proxy throw errors, so the proxy can't be confused with the target that way. So I guess I can add checkin-needed.

Pushed by cbook@mozilla.com: https://hg.mozilla.org/integration/fx-team/rev/2e03460fc927 When inspecting a proxy, show the [[ProxyHandler]] and [[ProxyTarget]] instead of executing traps. r=jlong