arrayMapIdentities

Reports using .flatMap() with an identity function that returns its argument unchanged.

✅ This rule is included in the ts logical and logicalStrict presets.

Using .flatMap() with a callback that returns its argument unchanged is equivalent to calling .flat(). The .flat() method is more concise and clearly expresses the intent to flatten an array.

Examples

❌ Incorrect

const result = values.flatMap((value) => value);

const result = values.flatMap((item) => {
  return item;
});

const result = values.flatMap(function (element) {
  return element;
});

✅ Correct

const result = values.flat();

const result = values.flatMap((value) => value.children);

const result = values.flatMap((value) => [value, ...value.extras]);

const result = values.flatMap((value) => transform(value));

Options

This rule is not configurable.

When Not To Use It

If you are maintaining a codebase that must support environments where .flat() is not available, you may need to disable this rule. Some older JavaScript environments do not support .flat() or .flatMap() and only polyfill .flatMap(), though this is rare in modern development.

Further Reading

• MDN: Array.prototype.flat()
• MDN: Array.prototype.flatMap()

Equivalents in Other Linters

• Biome: noFlatMapIdentity