@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.