@jest/expect

What is @jest/expect?

@jest/expect is a part of the Jest testing framework that provides a set of matchers to assert different conditions in your tests. It allows you to write assertions in a readable and expressive way, making it easier to test your JavaScript code.

What are @jest/expect's main functionalities?

Basic Matchers

Basic matchers allow you to test simple values. In this example, `toBe` is used to check if the result of `2 + 2` is `4`.

expect(2 + 2).toBe(4);

Truthiness

Matchers for different types of truthiness. Here, `toBeNull` checks if the value is `null`.

expect(null).toBeNull();

Numbers

Matchers for comparing numbers. In this example, `toBeGreaterThan` checks if `4` is greater than `3`.

expect(4).toBeGreaterThan(3);

Strings

Matchers for strings. This example uses `not.toMatch` to check that the string 'team' does not contain the letter 'I'.

expect('team').not.toMatch(/I/);

Arrays and Iterables

Matchers for arrays and iterables. Here, `toContain` checks if the array contains the element 'Alice'.

expect(['Alice', 'Bob']).toContain('Alice');

Exceptions

Matchers for exceptions. This example uses `toThrow` to check if a function throws an error with the message 'error'.

expect(() => { throw new Error('error'); }).toThrow('error');

Other packages similar to @jest/expect

chai

Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework. It provides a variety of assertion styles (should, expect, assert) and is highly extensible.

expect

Expect is a simple assertion library for JavaScript. It is often used with testing frameworks like Mocha. It provides a similar set of matchers to Jest's expect, but is more lightweight and less integrated.

should

Should is an expressive, readable, framework-agnostic assertion library. It extends the Object prototype with a single non-enumerable property, which allows for a very expressive and readable style.

README

@jest/expect

This package extends expect library with jest-snapshot matchers. It exports jestExpect object, which can be used as standalone replacement of expect.

The jestExpect function used in Jest. You can find its documentation on Jest's website.

Changelog

29.7.0

Features

• [create-jest] Add npm init / yarn create initialiser for Jest projects (#14465)
• [jest-validate] Allow deprecation warnings for unknown options (#14499)

Fixes

• [jest-resolver] Replace unmatched capture groups in moduleNameMapper with empty string instead of undefined (#14507)
• [jest-snapshot] Allow for strings as well as template literals in inline snapshots (#14465)
• [@jest/test-sequencer] Calculate test runtime if perStats.duration is missing (#14473)

Performance

• [@jest/create-cache-key-function] Cache access of NODE_ENV and BABEL_ENV (#14455)

Chore & Maintenance

• [jest-cli] Move internal config initialisation logic to the create-jest package (#14465)