Microsoft/TypeScript

Missing error if ClassDeclaration is used in Statement position

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#22,682 opened on Mar 18, 2018

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BugDomain: ParserEffort: CasualHelp Wanted

Description

TypeScript Version: 2.8.0-dev.20180318

Search Terms: class statement

Code

let codition = false as boolean;

if (condition)
    class C {} // runtime error if transpiled to ES2015 or above: Unexpected token class
if (condition)
    enum E { Foo } // should probably be an error?

new C(); // runtime error if transpiled to ES5, because 'C' is undefined
E.Foo; // runtime error: cannot read property 'Foo' of 'E'

Expected behavior:

Expected a compile error on class C and maybe on enum E

Actual behavior: no error for these declarations, instead I get a runtime error when targeting ES2015 or above.

Playground Link:

Related Issues:

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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
The issue involves the TypeScript compiler's parser or checker not raising an error when a class declaration appears as a statement after an if without braces. Investigate the parser logic in `src/compiler/parser.ts` where class declarations are parsed, and the checker in `src/compiler/checker.ts` for missing error reporting. The expected behavior is to emit an error similar to other disallowed statements. The issue is open and unassigned, making it a potential first contribution.
Missing error if ClassDeclaration is used in Statement position 路 Microsoft/TypeScript#22682 | Good First Issue