unicorn/no-array-fill-with-reference-type

fill() evaluates its argument once and puts that same value in every slot. With an object or an array, all the slots point to one object, and a change through one slot shows up in all of them.

The rule reports an object literal, an array literal, a class, and any new expression except new RegExp(). It also reports a const variable that's declared with one of these. Functions and regular expression literals aren't reported.

Configure

Off by default. To turn it on:

[lint.rules]
"unicorn/no-array-fill-with-reference-type" = "error" # or "warn"

Examples

Reported:

const grid = new Array(3).fill([]);

grid[0].push("x"); // [["x"], ["x"], ["x"]]

const empty = { visits: 0 };
const week = new Array(7).fill(empty);

Not reported:

const grid = Array.from({ length: 3 }, () => []);

grid[0].push("x"); // [["x"], [], []]

const week = Array.from({ length: 7 }, () => ({ visits: 0 }));
const totals = new Array(7).fill(0);