jest/prefer-strict-equal

toEqual() skips properties that are undefined, treats a hole in an array like undefined, and doesn't compare the two objects' classes. So expect({ name: "Ada", email: undefined }).toEqual({ name: "Ada" }) passes. toStrictEqual() checks all three, and that test fails.

--fix-suggestions renames the matcher. --fix doesn't, because a test that passes with toEqual() can fail with toStrictEqual().

Configure#

Off by default. To turn it on:

[lint.rules]
"jest/prefer-strict-equal" = "error" # or "warn"

Examples#

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

test("creates a user", () => {
  expect(createUser("Ada")).toEqual({ name: "Ada" });
});

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

test("creates a user", () => {
  expect(createUser("Ada")).toStrictEqual({ name: "Ada" });
});

When not to use it#

If your code builds objects with optional properties set to undefined, or you compare class instances with plain objects on purpose, toEqual() is the matcher you want.