For-Loop Variable Not Used in Condition or Update

ID

javascript.for_loop_var_not_used

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

JavaScript

Tags

control-flow, copy-paste, reliability

Description

Reports a for loop whose initialiser declares one or more variables that are referenced in <em>neither</em> the condition nor the update expression:

for (let i = 0; n < 10; n++) {       // i never used in cond/update
  doStuff();
}

for (let row = 0; col < width; col++) {   // wrong loop variable name
  ...
}

Rationale

Almost every loop of this shape is a copy-paste leftover or a half-finished rename: the developer started writing the header with one variable name and finished with another. The body may still terminate (via a break, an exception, …​) but the visible loop relationship is broken — readers can no longer see, from the header alone, what makes the loop progress.

Remediation

Make the condition and the update reference the loop variable:

for (let i = 0; i < 10; i++) {
  doStuff();
}

If the loop does not need a counter (for-of, for-in, while), use the matching construct:

for (const item of items) { ... }
while (cond) { ... }

Notes

The rule fires only when:

  • the initialiser declares at least one named variable (var, let, or const),

  • the condition is non-empty,

  • the update is non-empty,

  • and none of the declared variables appears as an Identifier in either the condition or the update.

Loops with no condition (for(;;)) or no update are explicit infinite-loop / break-from-body patterns and are not reported.

References