fix(forEach): fix a temporal dead zone issue in forEach. #2474
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src/Observable.ts
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| const subscription = this.subscribe((value) => { | ||
| // Must be explicitly assigned to avoid RefernceError when accessing | ||
| // subscription below in the closure due to Temporal Dead Zone. | ||
| let subscription: Subscription = undefined; |
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let subscription: Subscription;
According to the ES6 spec and the implementation in V8, accessing a lexically scoped construct like const or let its declaration has finished is a ReferenceError. Unlike var, it is not implicitly undefined in the entire function scope. Because the closure handed to subscribe is immediately invoked and accesses `subscription` before it is assigned, this causes a ReferenceError in compliant engines. The error is only triggered when running in plain ES6, i.e. without transpilation.
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Curiousity question: how previous code can be executed without transpilation to hit those errorneus cases? |
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@kwonoj if you run the code in an ES6 VM, it'll fail. But you can repro this much more easily locally: > function call(c) { return c(); }
undefined
> function undef() { const foo = call(() => foo ? 1 : 2); }
undefined
> undef()
ReferenceError: foo is not defined
at call (repl:1:55)
at call (repl:1:38)
at undef (repl:1:44)
at repl:1:1
at ContextifyScript.Script.runInThisContext (vm.js:23:33)
at REPLServer.defaultEval (repl.js:340:29)
at bound (domain.js:280:14)
at REPLServer.runBound [as eval] (domain.js:293:12)
at REPLServer.onLine (repl.js:537:10)
at emitOne (events.js:101:20)
> function undef() { let foo; foo = call(() => foo ? 1 : 2); }
undefined
> undef()
undefined |
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Ah, question was since code is TS anyway, it should be transpiled via compiler so curious how did original could be executed. Just curiousity question though. |
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If you transpile TypeScript with target language ES6, you get ES6 :-)
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Oh thought compiler does something like hoisting automatically. Thx for clarification. |
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@kwonoj can you merge? I don't have write access. |
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No, I intentionally left this to get second eye and release planning. /cc @benlesh |
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thanks @mprobst! |
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This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs. |
According to the ES6 spec and the implementation in V8, accessing a lexically scoped construct like const or let before it is assigned is a ReferenceError. Unlike var, it is not implicitly undefined. Because the closure handed to subscribe is immediately invoked and accesses
subscriptionbefore it is assigned, this causes a ReferenceError in compliant engines.The error is only triggered when running in plain ES6, i.e. without transpilation.