biome style/useTrimStartEnd: fix changes program behavior

Hypothesis Nemotron 3 Super predicted to fire fired 1×

numeric/string coercion: + vs template literal, objects with toString/valueOf, Symbol, BigInt

Program

Output

(no stdout)
TypeError: obj.trimLeft is not a function

(no stdout)
TypeError: obj.trimStart is not a function

Judge Nemotron 3 Ultra advisory intended-change · low

The rule intentionally replaces the deprecated aliases trimLeft/trimRight with the standard trimStart/trimEnd methods. The program calls trimLeft on a plain object that lacks either method, so both the original and fixed code throw a TypeError; the only difference is the method name in the error message, which directly reflects the documented rewrite.

Explanation Nemotron 3 Super unverified

The original method name trimLeft does not exist on objects; it was incorrectly assumed to be available like on strings. Renaming it to trimStart preserves the same incorrect assumption—both are undefined for plain objects—so the TypeError persists unchanged. The behavior differs only in the method name referenced in the error message, but the underlying semantics (attempting to call a non-existent method on an object) remain identical, making the rewrite ineffective at fixing the core issue. No actual behavioral change occurs beyond the error’s label.

Reproduce

file

prog.js (the before program)

minimized program

r1-p004.js

evidence sheet

r1-p004.md

regression fixture

r1-p004.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 · style/useTrimStartEnd deprecated string methods · style/useTrimStartEnd object trimLeft usage

no duplicate or related report 20 hits triaged

Details

program

Coercion Interactions

hazard

numeric/string coercion: + vs template literal, objects with toString/valueOf, Symbol, BigInt

oracle

stderr before: TypeError: obj.trimLeft is not a function stderr after: TypeError: obj.trimStart is not a function

run

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

--- orig.js	2026-10-17 17:07:30.919672857 -0400
+++ fixed.js	2026-10-17 17:07:31.183675697 -0400
@@ -1,4 +1,4 @@
 let obj = {
   toString: () => '   abc   '
 };
-console.log((obj).trimLeft() + 'def');
+console.log((obj).trimStart() + 'def');

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();
 ```