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JS: Improve useless-expression query to avoid duplicate alerts on compound expressions #19579

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JS: Improve useless-expression query to avoid duplicate alerts on compound expressions #19579

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@Napalys Napalys commented May 26, 2025 •
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This PR improves the js/useless-expression query by adding logic to avoid flagging compound expressions that may contain sub-expressions with side effects.

@Napalys Napalys marked this pull request as ready for review May 26, 2025 11:00
@Copilot Copilot AI review requested due to automatic review settings May 26, 2025 11:00
@Napalys Napalys requested a review from a team as a code owner May 26, 2025 11:00
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Pull Request Overview

Enhance the isDomProperty predicate to detect property reads on DOM nodes (e.g., offsetHeight, clientWidth) and reduce false positives in the js/useless-expression query.

  • Extend isDomProperty to also match property names accessed via data-flow property reads.
  • Add a test case validating layout-affecting reads on DOM elements.
  • Record the change in the project’s change notes.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File Description
javascript/ql/lib/Expressions/DOMProperties.qll Extend isDomProperty predicate with a DataFlow::SourceNode branch.
javascript/ql/test/query-tests/Expressions/ExprHasNoEffect/dom.js New test verifying that reads like offsetHeight and clientWidth are side-effectful.
javascript/ql/lib/change-notes/2025-05-26-dom-property-access.md Add change note documenting the enhancement to isDomProperty.
Comments suppressed due to low confidence (1)

javascript/ql/lib/Expressions/DOMProperties.qll:14

  • [nitpick] The variable name domNode implies a DOM AST node but actually refers to a data-flow source; consider renaming it to sourceNode or propSourceNode for clearer intent.
exists(DataFlow::SourceNode domNode | isDomNode(domNode) |

@Napalys Napalys marked this pull request as draft May 27, 2025 08:53
@Napalys Napalys force-pushed the js/dom_property_access branch 3 times, most recently from 201ee08 to 59fe03f Compare May 30, 2025 14:30
@Napalys Napalys force-pushed the js/dom_property_access branch from 59fe03f to 1f256ab Compare June 2, 2025 12:59
@Napalys Napalys changed the title JS: Enhance isDomProperty JS: Improve useless-expression query to avoid duplicate alerts on compound expressions Jun 2, 2025
@Napalys Napalys marked this pull request as ready for review June 2, 2025 13:07
asgerf
asgerf reviewed Jun 2, 2025
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javascript/ql/lib/Expressions/ExprHasNoEffect.qll Outdated
e instanceof SeqExpr
or
e instanceof ParExpr and
not e.stripParens() instanceof FunctionExpr
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Why this last condition about functions? Deserves a comment at least.

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I added this to handle cases like this one, otherwise we would lose the ability to detect such useless functions. I’ve also added a comment in commit 15b1dae. Hope that makes sense!

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But why is the function expression itself not flagged in this case?

We decided to alert on the innermost expression, as opposed to the outermost one. But this makes an exception for function expressions; for function expressions we now flag the enclosing parenthesis. I just don't see why there should be a special case for function expressions.

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@Napalys Napalys Jun 3, 2025 •
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That’s fair! It was silly of me to try fixing the problem at the surface 🤦 instead of tackling the root cause. What do you think of the approach I took in this chain of commits?
bf48b59 -> 46b5ded -> aac56e0

@Napalys Napalys requested a review from asgerf June 2, 2025 15:39
@Napalys Napalys force-pushed the js/dom_property_access branch from 15b1dae to aac56e0 Compare June 3, 2025 13:27
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