facebookarchive/prepack

Lexical declarations in top-level lexical scope are omitted

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#1905 opened on May 8, 2018

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 (3 comments) (1 reaction) (0 assignees)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.

Contributor guide

Tech stack
javascript
Domain
toolingbuild system
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
JavaScript scoping rulesPrepack dead code elimination
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
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.