unicorn/prefer-array-flat-map

.map().flat() builds an array of arrays only to take it apart again. .flatMap() does both in one pass, without the array in between.

--fix rewrites .map(fn).flat() to .flatMap(fn).

Configure#

Off by default. To turn it on:

[lint.rules]
"unicorn/prefer-array-flat-map" = "error" # or "warn"

Examples#

const tags = posts.map(post => post.tags).flat();
const words = lines.map(line => line.split(" ")).flat(1);

const tags = posts.flatMap(post => post.tags);
const words = lines.flatMap(line => line.split(" "));

.flatMap() flattens one level, so a deeper .flat() isn't reported:

const cells = tables.map(table => table.rows).flat(2);

Children from React has a map but no flatMap, so it's left alone:

const items = React.Children.map(children, child => child.props.items).flat();

A .filter() before a .flatMap() that returns an array literal of two or more elements is also reported. The .flatMap() callback can return [] for the elements you want to skip. --fix-suggestions makes this change.

const points = lines.filter(line => line.visible).flatMap(line => [line.start, line.end]);

const points = lines.flatMap(line => line.visible ? [line.start, line.end] : []);