noParametersOnlyUsedInRecursion

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JavaScript (and super languages)

Summary

Rule available since: v2.3.3
Diagnostic Category: lint/nursery/noParametersOnlyUsedInRecursion
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Inspired from only_used_in_recursion

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "noParametersOnlyUsedInRecursion": "error"
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      }
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    }
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  }
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}

Description

Disallow function parameters that are only used in recursive calls.

A parameter that is only passed to recursive calls is effectively unused and can be removed or replaced with a constant, simplifying the function.

This rule is inspired by Rust Clippy’s only_used_in_recursion lint.

Examples

Invalid

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function factorial(n, acc) {
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    if (n === 0) return 1;
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    return factorial(n - 1, acc);
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}

code-block.js:1:23 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter is only used in recursive calls.
  
  > 1 │ function factorial(n, acc) {
      │                       ^^^
    2 │     if (n === 0) return 1;
    3 │     return factorial(n - 1, acc);
  
  ℹ Parameters that are only used in recursive calls are effectively unused and can be removed.
  
  ℹ If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
  
  ℹ Unsafe fix: If this is intentional, prepend acc with an underscore.
  
    1   │ - function·factorial(n,·acc)·{
      1 │ + function·factorial(n,·_acc)·{
    2 2 │       if (n === 0) return 1;
    3   │ - ····return·factorial(n·-·1,·acc);
      3 │ + ····return·factorial(n·-·1,·_acc);
    4 4 │   }
    5 5 │

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function countdown(n, step) {
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    if (n === 0) return 0;
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    return countdown(n - step, step);
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}

code-block.js:1:23 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter is only used in recursive calls.
  
  > 1 │ function countdown(n, step) {
      │                       ^^^^
    2 │     if (n === 0) return 0;
    3 │     return countdown(n - step, step);
  
  ℹ Parameters that are only used in recursive calls are effectively unused and can be removed.
  
  ℹ If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
  
  ℹ Unsafe fix: If this is intentional, prepend step with an underscore.
  
    1   │ - function·countdown(n,·step)·{
      1 │ + function·countdown(n,·_step)·{
    2 2 │       if (n === 0) return 0;
    3   │ - ····return·countdown(n·-·step,·step);
      3 │ + ····return·countdown(n·-·_step,·_step);
    4 4 │   }
    5 5 │

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class Counter {
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    count(n, acc) {
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        if (n === 0) return 0;
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        return this.count(n - 1, acc);
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    }
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}

code-block.js:2:14 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter is only used in recursive calls.
  
    1 │ class Counter {
  > 2 │     count(n, acc) {
      │              ^^^
    3 │         if (n === 0) return 0;
    4 │         return this.count(n - 1, acc);
  
  ℹ Parameters that are only used in recursive calls are effectively unused and can be removed.
  
  ℹ If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
  
  ℹ Unsafe fix: If this is intentional, prepend acc with an underscore.
  
    1 1 │   class Counter {
    2   │ - ····count(n,·acc)·{
      2 │ + ····count(n,·_acc)·{
    3 3 │           if (n === 0) return 0;
    4   │ - ········return·this.count(n·-·1,·acc);
      4 │ + ········return·this.count(n·-·1,·_acc);
    5 5 │       }
    6 6 │   }

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function fn(n, acc) {
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    if (n === 0) return 0;
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    return fn(n - 1, acc || 0);
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}

code-block.js:1:16 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter is only used in recursive calls.
  
  > 1 │ function fn(n, acc) {
      │                ^^^
    2 │     if (n === 0) return 0;
    3 │     return fn(n - 1, acc || 0);
  
  ℹ Parameters that are only used in recursive calls are effectively unused and can be removed.
  
  ℹ If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
  
  ℹ Unsafe fix: If this is intentional, prepend acc with an underscore.
  
    1   │ - function·fn(n,·acc)·{
      1 │ + function·fn(n,·_acc)·{
    2 2 │       if (n === 0) return 0;
    3   │ - ····return·fn(n·-·1,·acc·||·0);
      3 │ + ····return·fn(n·-·1,·_acc·||·0);
    4 4 │   }
    5 5 │

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class Counter {
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    count(n, acc) {
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        if (n === 0) return 0;
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        return this?.count(n - 1, acc);
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    }
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}

code-block.js:2:14 lint/nursery/noParametersOnlyUsedInRecursion  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter is only used in recursive calls.
  
    1 │ class Counter {
  > 2 │     count(n, acc) {
      │              ^^^
    3 │         if (n === 0) return 0;
    4 │         return this?.count(n - 1, acc);
  
  ℹ Parameters that are only used in recursive calls are effectively unused and can be removed.
  
  ℹ If the parameter is needed for the recursion to work, consider if the function can be refactored to avoid it.
  
  ℹ Unsafe fix: If this is intentional, prepend acc with an underscore.
  
    1 1 │   class Counter {
    2   │ - ····count(n,·acc)·{
      2 │ + ····count(n,·_acc)·{
    3 3 │           if (n === 0) return 0;
    4   │ - ········return·this?.count(n·-·1,·acc);
      4 │ + ········return·this?.count(n·-·1,·_acc);
    5 5 │       }
    6 6 │   }

Valid

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function factorial(n, acc) {
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    if (n === 0) return acc;
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    return factorial(n - 1, acc * n);
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}

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function countdown(n, step) {
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    console.log(step);
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    if (n === 0) return 0;
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    return countdown(n - step, step);
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}

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function fn(n, threshold) {
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    if (n > threshold) return n;
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    return fn(n + 1, threshold);
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}