💅 noUnnecessaryConditions: new RegExp(...).exec() is treated as non-nullable

[shbernal]

Environment information

Details

CLI:
  Version:                      2.5.7
  Color support:                false

Platform:
  CPU Architecture:             x86_64
  OS:                           windows

Environment:
  BIOME_DISTRIBUTION:                npm
  BIOME_LOG_PATH:                    unset
  BIOME_LOG_PREFIX_NAME:             unset
  BIOME_LOG_LEVEL:                   unset
  BIOME_LOG_KIND:                    unset
  BIOME_CONFIG_PATH:                 unset
  BIOME_THREADS:                     unset
  BIOME_WATCHER_KIND:                unset
  BIOME_WATCHER_POLLING_INTERVAL:    unset
  NO_COLOR:                     1
  TERM:                         xterm-256color
  JS_RUNTIME_VERSION:           v24.16.0
  JS_RUNTIME_NAME:              node
  NODE_PACKAGE_MANAGER:         unset

Biome Configuration:
  Status:                       Loaded successfully.
  Path:                         biome.json
  Formatter enabled:            true
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  Assist enabled:               true
  VCS enabled:                  false
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Linter:
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  JSON enabled:                 unset
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  Enabled rules:
    suspicious/noUnnecessaryConditions

Workspace:
  Open Documents:               0

Rule name

suspicious/noUnnecessaryConditions

Playground link

https://biomejs.dev/playground/?lintRules=all&tab=diagnostics&code=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%2BACgAWwBeADwAXQAqACkAYAApAC4AZQB4AGUAYwAoAGkAbgBwAHUAdAApAD8ALgBbADEAXQAgAD8APwAgACcAJwA7AAoAfQAKAA%3D%3D

Expected result

No diagnostic. RegExp.prototype.exec returns RegExpExecArray | null, so the ?. is required however the RegExp was constructed.

Reproduction

// `RegExp.prototype.exec` returns `RegExpExecArray | null`, so `?.` is required.
export function capture(input: string): string | undefined {
  return /a(b)/.exec(input)?.[1];
}

export function captureDynamic(input: string, tag: string): string {
  return new RegExp(`<${tag}>([^<]*)`).exec(input)?.[1] ?? '';
}

b-regexp-exec.ts:7:10 lint/suspicious/noUnnecessaryConditions ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  × Unnecessary optional chaining.

  > 7 │   return new RegExp(`<${tag}>([^<]*)`).exec(input)?.[1] ?? '';
      │          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  i Replace ?. with ..

  i The receiver is guaranteed to be non-nullish.

Found 1 error.

The suggested fix is unsafe: taking it turns a non-match from undefined into a TypeError at runtime.

Analysis

The two functions differ only in how the pattern is built, and only the second is flagged:

• /a(b)/.exec(input)?.[1] — not flagged, correct.
• new RegExp(...).exec(input)?.[1] — flagged.

So the regex literal carries a correctly-nullable exec while the RegExp constructor call does not — it looks like the constructed-instance path loses the | null from the return type rather than the rule mishandling the chain.

Possibly a facet of #5977 (globals resolved from hardcoded definitions rather than real lib.d.ts), but the literal/constructor asymmetry suggests it may be fixable ahead of that work.

Context

Found while trialling the type-aware rules on a TypeScript-first OOXML/xlsx library. Dynamically built tag patterns are common in its test assertions, so this is 10 of the 148 noUnnecessaryConditions diagnostics on that tree.

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✅ Organic activity

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_{This is an automated analysis by AgentScan}