biome style/useTrimStartEnd: fix changes program behavior

Hypothesis Nemotron 3 Super predicted to fire fired 1×

exceptions thrown inside the code the fix removes or rewrites

Program

Output

TypeError: Cannot read properties of null (reading 'trimLeft')

TypeError: Cannot read properties of null (reading 'trimStart')

Judge Nemotron 3 Ultra advisory introspection-only · low

The program throws a TypeError because `s` is null, and the error message includes the method name that was called. The fix changes `trimLeft` to its alias `trimStart`, so the only difference is the method name quoted in the error message, which is source-text introspection.

Explanation Nemotron 3 Super unverified

The original call `s.trimLeft()` attempts to access property `trimLeft` on `null`, causing a TypeError with message mentioning `trimLeft`. After rewriting to `s.trimStart()`, the same error occurs but now references `trimStart` in the message string (`Cannot read properties of null (reading 'trimStart')`). Although both throw identical exception types, the error message differs because it includes the accessed property name. Since observable output (the printed message) changes, the transformation alters program behavior and is not semantics‑preserving.

Reproduce

file

prog.js (the before program)

minimized program

r1-p005.js

evidence sheet

r1-p005.md

regression fixture

r1-p005.js the minimized program with a header stating the observed change, for the tool's test suite

node prog.js
bunx @biomejs/biome lint --only=style/useTrimStartEnd --write prog.js
node prog.js

Upstream search GitHub issues Tavily Nemotron 3 Super + Ultra

useTrimStartEnd fix · style/useTrimStartEnd trimLeft method call · biome lint rule trimStart replacement · null object method invocation error

no duplicate or related report 18 hits triaged

Details

program

Exception Safety

hazard

exceptions thrown inside the code the fix removes or rewrites

oracle

stdout before: TypeError: Cannot read properties of null (reading 'trimLeft') stdout after: TypeError: Cannot read properties of null (reading 'trimStart')

run

2026-10-17T21-07-19-761Z-tpod-biome-style_useTrimStartEnd

--- orig.js	2026-10-17 17:07:31.355677548 -0400
+++ fixed.js	2026-10-17 17:07:31.629680495 -0400
@@ -1,6 +1,6 @@
 let s = null;
 try {
-  console.log(s.trimLeft());
+  console.log(s.trimStart());
 } catch (e) {
   console.log(e.name + ': ' + e.message);
 }

Summary

- Name: useTrimStartEnd
- Fix: safe
- Default severity: info
- Available from version: 1.9.0
- Diagnostic category: lint/style/useTrimStartEnd


Description

 Enforce the use of `String.trimStart()` and `String.trimEnd()` over `String.trimLeft()` and `String.trimRight()`.

 While `String.trimLeft()` and `String.trimRight()` are aliases for `String.trimStart()` and `String.trimEnd()`,
 only using the latter pair ensures consistency and is preferable for their direction-independent wording.

 Note that `String.trimStart()` and `String.trimEnd()` methods do not take any parameters. Any arguments passed to these methods will be ignored.
 See the MDN documentation for more details:
 - [String.prototype.trimStart()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trimStart)
 - [String.prototype.trimEnd()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trimEnd)

Examples

Invalid

 ```js,expect_diagnostic
 const foo = bar.trimLeft();
 ```

 ```js,expect_diagnostic
 const foo = bar.trimRight();
 ```

Valid

 ```js
 const foo = bar.trimStart();
 ```

 ```js
 const foo = bar.trimEnd();
 ```