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.

const 
const result: number[]
result = 
const values: number[][]
values.
Array<number[]>.flatMap<number, undefined>(callback: (this: undefined, value: number[], index: number, array: number[][]) => number | readonly number[], thisArg?: undefined): number[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap((
value: number[]
value) => 
value: number[]
value);

const 
const result: number[]
result = 
const values: number[][]
values.
Array<number[]>.flatMap<number, undefined>(callback: (this: undefined, value: number[], index: number, array: number[][]) => number | readonly number[], thisArg?: undefined): number[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap((
item: number[]
item) => {
  return 
item: number[]
item;
});

const 
const result: number[]
result = 
const values: number[][]
values.
Array<number[]>.flatMap<number, undefined>(callback: (this: undefined, value: number[], index: number, array: number[][]) => number | readonly number[], thisArg?: undefined): number[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap(function (
element: number[]
element) {
  return 
element: number[]
element;
});

const 
const result: number[]
result = 
const values: number[][]
values.
Array<number[]>.flat<number[][], 1>(this: number[][], depth?: 1 | undefined): number[]
Returns a new array with all sub-array elements concatenated into it recursively up to the
specified depth.
@param ― depth The maximum recursion depth
flat();

const 
const result: string[]
result = 
const values: {
    children: string[];
}[]
values.
Array<{ children: string[]; }>.flatMap<string, undefined>(callback: (this: undefined, value: {
    children: string[];
}, index: number, array: {
    children: string[];
}[]) => string | readonly string[], thisArg?: undefined): string[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap((
value: {
    children: string[];
}
value) => 
value: {
    children: string[];
}
value.
children: string[]
children);

const 
const result: (number | {
    extras: number[];
})[]
result = 
const values: {
    extras: number[];
}[]
values.
Array<{ extras: number[]; }>.flatMap<number | {
    extras: number[];
}, undefined>(callback: (this: undefined, value: {
    extras: number[];
}, index: number, array: {
    extras: number[];
}[]) => number | {
    extras: number[];
} | readonly (number | {
    extras: number[];
})[], thisArg?: undefined): (number | {
    extras: number[];
})[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap((
value: {
    extras: number[];
}
value) => [
value: {
    extras: number[];
}
value, ...
value: {
    extras: number[];
}
value.
extras: number[]
extras]);

const 
const result: number[]
result = 
const values: number[]
values.
Array<number>.flatMap<number, undefined>(callback: (this: undefined, value: number, index: number, array: number[]) => number | readonly number[], thisArg?: undefined): number[]
Calls a defined callback function on each element of an array. Then, flattens the result into
a new array.
This is identical to a map followed by flat with depth 1.
@param ― callback A function that accepts up to three arguments. The flatMap method calls the
callback function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the callback function. If
thisArg is omitted, undefined is used as the this value.
flatMap((
value: number
value) => 
const transform: (value: number) => number[]
transform(
value: number
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.

Equivalents in Other Linters

Biome: noFlatMapIdentity

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arrayMapIdentities

Examples

Options

When Not To Use It

Further Reading

Equivalents in Other Linters