jest/prefer-to-have-length

When expect(users.length).toBe(2) fails, the message shows two numbers. expect(users).toHaveLength(2) also prints the value it measured, so you see which items are missing or extra.

The rule reports .length inside expect() when the matcher is toBe(), toEqual(), or toStrictEqual().

--fix rewrites the assertion.

Configure

Off by default. To turn it on:

[lint.rules]
"jest/prefer-to-have-length" = "error" # or "warn"

Examples

Reported:

import { expect, test } from "@jest/globals";

test("finds the active users", () => {
  const users = findUsers({ active: true });
  expect(users.length).toBe(2);
  expect(users[0].roles.length).not.toEqual(0);
});

Not reported:

import { expect, test } from "@jest/globals";

test("finds the active users", () => {
  const users = findUsers({ active: true });
  expect(users).toHaveLength(2);
  expect(users[0].roles).not.toHaveLength(0);
  expect(users.length).toBeGreaterThan(1);
});