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jest/prefer-expect-assertions Style

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What it does

Enforces that every test has either expect.assertions(<number>) or expect.hasAssertions() as its first expression.

Why is this bad?

Without explicit assertion counts, tests with asynchronous code, callbacks, or loops may pass even if some expect calls are never reached, silently hiding bugs.

Examples

Examples of incorrect code for this rule:

test("no assertions", () => {
  // ...
});
test("assertions not first", () => {
  expect(true).toBe(true);
  // ...
});

Examples of correct code for this rule:

test("with assertion count", () => {
  expect.assertions(1);
  expect(true).toBe(true);
});
test("with hasAssertions", () => {
  expect.hasAssertions();
  expect(true).toBe(true);
});

///Examples of incorrect code with { "onlyFunctionsWithAsyncKeyword": true }:

test("fetches data", async () => {
  const data = await fetchData();
  expect(data).toBe("peanut butter");
});

Examples of correct code with { "onlyFunctionsWithAsyncKeyword": true }:

test("fetches data", async () => {
  expect.assertions(1);
  const data = await fetchData();
  expect(data).toBe("peanut butter");
});

Examples of incorrect code with { "onlyFunctionsWithExpectInLoop": true }:

test("all numbers are greater than zero", () => {
  for (const number of getNumbers()) {
    expect(number).toBeGreaterThan(0);
  }
});

Examples of correct code with { "onlyFunctionsWithExpectInLoop": true }:

test("all numbers are greater than zero", () => {
  expect.hasAssertions();
  for (const number of getNumbers()) {
    expect(number).toBeGreaterThan(0);
  }
});

Examples of incorrect code with { "onlyFunctionsWithExpectInCallback": true }:

test("callback test", () => {
  fetchData((data) => {
    expect(data).toBe("peanut butter");
  });
});

Examples of correct code with { "onlyFunctionsWithExpectInCallback": true }:

test("callback test", () => {
  expect.assertions(1);
  fetchData((data) => {
    expect(data).toBe("peanut butter");
  });
});

Configuration

This rule accepts a configuration object with the following properties:

onlyFunctionsWithAsyncKeyword

type: boolean

default: false

onlyFunctionsWithExpectInCallback

type: boolean

default: false

onlyFunctionsWithExpectInLoop

type: boolean

default: false

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "plugins": ["jest"],
  "rules": {
    "jest/prefer-expect-assertions": "error"
  }
}

Config (oxlint.config.ts)

import { defineConfig } from "oxlint";

export default defineConfig({
  plugins: ["jest"],
  rules: {
    "jest/prefer-expect-assertions": "error",
  },
});

CLI

oxlint --deny jest/prefer-expect-assertions --jest-plugin

Version

This rule was added in v1.62.0.

References

• Rule Source
• Upstream rule docs
• Open rule in Oxc Playground