LGTM.com - false positive - TypeScript

[AlexTereshenkov]

Description of the false positive

Useless assignment to local variable alert is raised as part of the import statement.

URL to the alert on the project page on LGTM.com

https://lgtm.com/projects/g/kyranet/Skyra/snapshot/0e406afb893eb5bc2aca7afca3177a7f68f6bc6e/files/src/tasks/reminder.ts?sort=name&dir=ASC&mode=heatmap

Maybe related to a previous issue. The issue may have been fixed with #1076.

It looks like we have a problem with nameless TypeScript classes (off by one, perhaps?).
js/useless-assignment-to-local flags C2 in the following code when it is in a file with .ts extension, but not when the file has a .js extension:

var C1 = global.C1;
var C2 = global.C2;

class C extends C1 {}
export default class extends C2 {}

See https://git.semmle.com/esben/ql/commit/2dfd0f98fc316e8ced6e5e8010eae35cd54f7bcf

[erik-krogh]

It seems to be related to SSA/BasicBlocks.

The following simple query returns a different result for the .js vs. the .ts file from above.

from SsaSourceVariable v, ReachableBasicBlock bb, VarUse u
where v = u.getVariable() and bb.getANode() = u
select v

Interrestingly, replacing ReachableBasicBlock with BasicBlock and the results are the same for both files.

I'll debug some more tomorrow.

[asger-semmle]

The root cause of this kind of issue is almost always in the extractor, since the everything in the QL layer is shared between JS and TS.

Looking at the AST in astexplorer.net, it seems the class expression is represented as a ClassDeclaration in the TypeScript AST, and we don't convert it correctly in TypeScriptASTConverter.

[erik-krogh]

Looking at the AST in astexplorer.net, it seems the class expression is represented as a ClassDeclaration in the TypeScript AST, and we don't convert it correctly in TypeScriptASTConverter.

You're right. The JavaScript extractor produces a ClassExpr while the TypeScript extractor produces a ClassDeclStmt for the class extends C2 {} expression.

[kyranet]

Has this been landed into production yet? I am getting errors tirelessly.

[xiemaisi]

No, this fix has not been deployed yet.