facebookarchive/prepack

Lexical declarations in top-level lexical scope are omitted

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#1 905 ouverte le 8 mai 2018

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 (3 commentaires) (1 réaction) (0 assignés)JavaScript (14 268 stars) (520 forks)batch import
bughelp wantedlevel 2 (easy)serializer

Description

const f = 0; is considered by Prepack to be totally dead. But it's not: the top-level lexical scope is shared across scripts, just below the global scope. So while that may not be a global declaration, in that it does not create a property of the global object, it is available to other code on the page just as if it had been var instead of const; other scripts can write console.log(f) to print 0. In that sense it is not safe to eliminate.

This may or may not be a bug, depending on your perspective. I'm just not sure what things prepack considers dead: { const f = 0; } definitely is, var f = 0; definitely isn't, but this is kind of in-between.

Guide contributeur

Stack technique
javascript
Domaine
toolingbuild system
Type d'issue
bug
DifficultéDifficulté estimée pour un nouveau contributeur, de 1 pour un très petit changement à 5 pour un travail expert.
4
Temps estiméFourchette de temps approximative pour investiguer, implémenter, tester et préparer une pull request.
half day
Statut d'activitéDisponibilité apparente de l'issue : fraîche, active, ancienne, bloquée ou en attente d'un mainteneur.
stale
ClartéClarté avec laquelle l'issue explique le changement attendu, les critères d'acceptation et la prochaine étape.
needs investigation
Prérequis
JavaScript scoping rulesPrepack dead code elimination
Accessibilité débutantScore de 1 à 100 estimant l'accessibilité de cette issue pour un premier contributeur.
20
Direction de recherche
Investigate how Prepack currently handles top level lexical declarations (const/let) vs var. Check if the issue references existing code in Prepack's scope analysis (e.g., Scope.js or similar). Review comment #4 if available. Determine whether the elimination decision is intentional or a bug by consulting maintainer responses or documentation.