Microsoft/TypeScript

"const" was transformed to "var" when target is "esnext"

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#59,877 opened on Sep 6, 2024

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 (3 comments) (0 reactions) (0 assignees)TypeScript (6,726 forks)batch import
BugDomain: JS EmitHelp Wanted

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Description

🔎 Search Terms

transform, const, var, target, ECMA, esnext

🕗 Version & Regression Information

  • This is the behavior in every version I tried from Version 5.4 to Version 5.7.0-dev.20240904

⏯ Playground Link

No response

💻 Code

export const cilBlurLinear : string [ ] = [  ,  ] 
const [ ,  ] = cilBlurLinear;

🙁 Actual behavior

The JS code generated by tsc is as follows:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.cilBlurLinear = void 0;
exports.cilBlurLinear = [,];
var ;

🙂 Expected behavior

“const” should be translated as "const" instead of "var", as the target in my configuration file is "esnext".

Additional information about the issue

tsconfig.json:

{
  "compilerOptions": {
    "target": "ESNext",
    "module": "esnext",
    "moduleResolution": "Node",
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "lib": ["es2023","dom"],
    "noEmitOnError":true,
    "force":true,
    "strict":true
  },
  "include": [
    "TScorpus/*"
  ],
  "exclude": [
    "node_modules"
  ]
}

Contributor guide

Research direction
Inspect the compiler's emit code for const destructuring patterns that skip elements. Identify why it emits 'var' instead of 'const' when target is ESNext. Check the binder or transformer logic for destructuring variable declarations with elisions.
Tech stack
typescript
Domain
build system
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60