vitest/prefer-comparison-matcher

expect(price < 100).toBe(true) compares first and only hands expect the result. When it fails, the message says that false isn't true, and you don't see the price. expect(price).toBeLessThan(100) prints both numbers.

The rule reports >, >=, < and <= inside expect() when the result is checked against true or false with toBe(), toEqual() or toStrictEqual().

--fix rewrites the assertion.

Configure#

Off by default. To turn it on:

[lint.rules]
"vitest/prefer-comparison-matcher" = "error" # or "warn"

Examples#

import { expect, test } from "vitest";

test("applies the discount", () => {
  const price = discounted(100);
  expect(price < 100).toBe(true);
  expect(price < 0).toBe(false);
});

import { expect, test } from "vitest";

test("applies the discount", () => {
  const price = discounted(100);
  expect(price).toBeLessThan(100);
  expect(price).toBeGreaterThanOrEqual(0);
});

The comparison matchers only take numbers and bigints, so the rule skips a comparison with a string literal or a template literal:

import { expect, test } from "vitest";

test("sorts after the first entry", () => {
  expect(lastName() > "Adams").toBe(true);
});

When not to use it#

The rule doesn't look at types, so it also reports comparisons of dates, or of strings held in variables. The comparison matchers throw on those values, and --fix rewrites the assertion anyway. This test is correct as written:

import { expect, test } from "vitest";

test("renewing extends the subscription", () => {
  expect(renewed.expiresAt > current.expiresAt).toBe(true);
});