Microsoft/TypeScript

Middle operand in comma operator list not properly checked for side-effect-freeness

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#33,646 opened on Sep 27, 2019

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BugDomain: Error MessagesHelp Wanted

Description

TypeScript Version: master at f304b81fa54285f Search Terms: comma side effects

Code

declare const obj: any;
console.log(`${JSON.stringify(obj), undefined, 2}`);

Expected behavior: error TS2695: Left side of comma operator is unused and has no side effects.

Actual behavior: No errors

Playground Link: http://www.typescriptlang.org/play/#code/CYUwxgNghgTiAEYD2A7AzgF3kgRgKwC54oUBPAbgChl0kIQA6CJAcwAoADAEgG8ApAMoB5AHINMMAJYoWkgGak2uPAEoANPACuKUHOkhgGgEwBfDiqpA

Related Issues: #10814

Contributor guide

Tech stack
typescript
Domain
backend
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Understanding of TypeScript compiler internalsFamiliarity with side effect analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the comma operator side effect checking in the TypeScript compiler. The related issue #10814 may provide context. Look at the checker file (likely checker.ts) where TS2695 is emitted to understand why the middle operand is not being checked. The fix may involve ensuring all operands in a comma expression are evaluated for side effect freeness.