useFlatMap (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/complexity/useFlatMap
This rule is recommended, meaning it is enabled by default.
This rule has a safe fix.
The default severity of this rule is information.
Sources:
- Same as unicorn/prefer-array-flat-map
- Same as map_flatten

How to configure

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{
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  "linter": {
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    "rules": {
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      "complexity": {
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        "useFlatMap": "error"
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      }
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    }
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  }
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}

Description

Promotes the use of .flatMap() when map().flat() are used together.

Using .flatMap() instead of map().flat() avoids the creation of an intermediate array.

Examples

Invalid

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const array = ["split", "the text", "into words"];
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array.map(sentence => sentence.split(' ')).flat();

code-block.js:2:1 lint/complexity/useFlatMap  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The call chain .map().flat() can be replaced with a single .flatMap() call.
  
    1 │ const array = ["split", "the text", "into words"];
  > 2 │ array.map(sentence => sentence.split(' ')).flat();
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Using .flatMap() avoids the creation of an intermediate array.
  
  ℹ Safe fix: Replace the chain with .flatMap().
  
    1 1 │   const array = ["split", "the text", "into words"];
    2   │ - array.map(sentence·=>·sentence.split('·')).flat();
      2 │ + array.flatMap(sentence·=>·sentence.split('·'));
    3 3 │

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const array = ["split", "the text", "into words"];
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array.map(sentence => sentence.split(' ')).flat(1);

code-block.js:2:1 lint/complexity/useFlatMap  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The call chain .map().flat() can be replaced with a single .flatMap() call.
  
    1 │ const array = ["split", "the text", "into words"];
  > 2 │ array.map(sentence => sentence.split(' ')).flat(1);
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Using .flatMap() avoids the creation of an intermediate array.
  
  ℹ Safe fix: Replace the chain with .flatMap().
  
    1 1 │   const array = ["split", "the text", "into words"];
    2   │ - array.map(sentence·=>·sentence.split('·')).flat(1);
      2 │ + array.flatMap(sentence·=>·sentence.split('·'));
    3 3 │

Valid

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const array = ["split", "the text", "into words"];
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array.map(sentence => sentence.split(' ')).flat(2);