noMismatchedMapKey (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.5
Diagnostic Category: lint/nursery/noMismatchedMapKey
This rule doesn’t have a fix.
The default severity of this rule is warning.
Sources:
- Inspired from unicorn/no-mismatched-map-key

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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        "noMismatchedMapKey": "error"
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      }
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    }
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  }
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}

Description

Disallow checking a map for one key and then using a different key.

After map.has(key) checks whether a key exists, the code that runs because of that check usually reads or writes the same key with map.get(key) or map.set(key, value). Using a different key there is almost always a copy-paste mistake.

This rule checks the branches of if statements, while loops, ternaries (? :), and the right side of && and || that run because of a has() check. It only reports simple cases where the keys are clearly different: two different literal values, or two different variables or properties.

A branch that is a block with more than one statement isn’t checked. A branch that assigns a new value to the map isn’t checked either, because the get() or set() call may use a different map.

Examples

Invalid

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const value = map.has(key) ? map.get(otherKey) : fallback;

code-block.js:1:38 lint/nursery/noMismatchedMapKey ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This get() call uses a different key than the has() check before it.
  
  > 1 │ const value = map.has(key) ? map.get(otherKey) : fallback;
      │                                      ^^^^^^^^
    2 │ 
  
  ℹ The map is checked for this key.
  
  > 1 │ const value = map.has(key) ? map.get(otherKey) : fallback;
      │                       ^^^
    2 │ 
  
  ℹ Checking one key and then using another is usually a copy-paste mistake, so this call may use the wrong entry.
  
  ℹ Use the same key in both calls.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

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if (!map.has(key)) {
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    map.set(otherKey, value);
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}

code-block.js:2:13 lint/nursery/noMismatchedMapKey ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This set() call uses a different key than the has() check before it.
  
    1 │ if (!map.has(key)) {
  > 2 │     map.set(otherKey, value);
      │             ^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ The map is checked for this key.
  
  > 1 │ if (!map.has(key)) {
      │              ^^^
    2 │     map.set(otherKey, value);
    3 │ }
  
  ℹ Checking one key and then using another is usually a copy-paste mistake, so this call may use the wrong entry.
  
  ℹ Use the same key in both calls.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

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function getUser(cache) {
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    if (cache.has("user")) {
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        return cache.get("users");
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    }
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}

code-block.js:3:26 lint/nursery/noMismatchedMapKey ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This get() call uses a different key than the has() check before it.
  
    1 │ function getUser(cache) {
    2 │     if (cache.has("user")) {
  > 3 │         return cache.get("users");
      │                          ^^^^^^^
    4 │     }
    5 │ }
  
  ℹ The map is checked for this key.
  
    1 │ function getUser(cache) {
  > 2 │     if (cache.has("user")) {
      │                   ^^^^^^
    3 │         return cache.get("users");
    4 │     }
  
  ℹ Checking one key and then using another is usually a copy-paste mistake, so this call may use the wrong entry.
  
  ℹ Use the same key in both calls.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.

Valid

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const value = map.has(key) ? map.get(key) : fallback;
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if (!map.has(key)) {
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    map.set(key, value);
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}
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if (map.has(key)) {
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    otherMap.get(otherKey);
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}