Chakracore: Symbol.for and usage as Map key do not work as expected

[psavur]

Try the following in console:

m1 = new Map();
m1.set(Symbol.for("n1"), "v1");
m1.get(Symbol.for("n1"));
undefined

Actual: undefined
Expected: "v1"

Same works in Chrome and returns "v1".
Symbol.for() should have created an entry in the global symbol registry.

Screenshot below

EDGE version:
Microsoft Edge 42.17134.1.0
Microsoft EdgeHTML 17.17134

Windows 10 Enterprise Eval, Version: 1803, OS Build: 17134.1

[kfarnung]

Looks like this was already fixed in newer versions:

## Source
m1 = new Map();
m1.set(Symbol.for("n1"), "v1");
print(m1.get(Symbol.for("n1")));

#### Chakra, JavaScriptCore, SpiderMonkey, V8, V8 --harmony
v1

I tried in a recent Windows Insider build and it seems to confirm:

[psavur]

Thats good that newer builds have this working.
Do you know when this would be released in public/GA Edge builds?

[kfarnung]

That's pretty bizarre, I tried on an 1803 machine (17134.191) and didn't repro at first, but on the second attempt I got a slightly weird behavior:

@boingoing @MSLaguana any ideas?

[boingoing]

Seems to work ok for me in the latest RS5 flight. I don't know when the bug was fixed, though. That behavior on 1803 doesn't look right. I wonder if that script was somehow executed on a different ThreadContext? The symbol global registry is per-ThreadContext.

[psavur]

I repeated using Symbol.for couple of times and see inconsistent behavior.
It sometimes allows me to set the same key twice in the map. This happens ~50% of the time.

I get a feeling Symbol.for() is having some issue where it sometimes does not see the symbols already created in the global registry.

I even went back to an older (by couple of months) Edge build on another Windows 10 instance and see similar behavior.

@kfarnung @boingoing

[MSLaguana]

I don't think that this can be an issue with running on a different thread; I tried this:

var s = Symbol.for("n1");
var m1 = new Map();
m1.set(s, "v1");
for(var i = 0; i < 1000; ++i) {
  var t = Symbol.for("n1");
  if (t !== s) {
    s2 = t;
    console.log("Found");
    break;
  }
  if (m1.get(t) !== m1.get(s)) {
    s2 = t;
    console.log("Get difference " + i);
    break;
  }
}

and that ends up printing "Get difference" with i either 0 or 1. Since it can happen with i=1 that says to me that m1.get isn't stable somehow.

[MSLaguana]

It looks like the bug was fixed by @sethbrenith in 1c0c67e01475aa3a83cba1e1312275ad59498076 when he tweaked how symbols were created.

[MSLaguana]

This will be fixed in the next release of edge. As a workaround in the current version of edge, if you only create a symbol once and keep re-using the same reference (e.g. instead of m1.set(Symbol.for("n1"),...) you did var sym = Symbol.for("n1"); m1.set(sym, …);) then I believe it should work as expected.

[psavur]

The above commit mentioned in @MSLaguana post was merged in January 2018. Do you know which version of Edge will have that merged fix? (fyi: To restate, this is still seen in version 42, the latest release)

Regarding the workaround, if I end up re-using the same reference to the Symbol by passing the reference around, then essentially it defeats the purpose of Symbol.for().

[MSLaguana]

It is fixed in current insider builds, so the next (non-security patch) release of edge should have the fix.

[psavur]

Would that release be version 43?

[boingoing]

I don't believe the Edge team has committed to the next version number, yet. It looks like current Insider flights of Windows include Edge version 44.