@std/expect

Overview Jump to heading#

This module provides Jest compatible expect assertion functionality.

import { expect } from "@std/expect";

const x = 6 * 7;
expect(x).toEqual(42);
expect(x).not.toEqual(0);

await expect(Promise.resolve(x)).resolves.toEqual(42);

Currently this module supports the following functions:

• Common matchers:
  • toBe
  • toEqual
  • toStrictEqual
  • toMatch
  • toMatchObject
  • toBeDefined
  • toBeUndefined
  • toBeNull
  • toBeNaN
  • toBeTruthy
  • toBeFalsy
  • toContain
  • toContainEqual
  • toHaveLength
  • toBeGreaterThan
  • toBeGreaterThanOrEqual
  • toBeLessThan
  • toBeLessThanOrEqual
  • toBeCloseTo
  • toBeInstanceOf
  • toThrow
  • toHaveProperty
• Mock related matchers:
  • toHaveBeenCalled
  • toHaveBeenCalledTimes
  • toHaveBeenCalledWith
  • toHaveBeenLastCalledWith
  • toHaveBeenNthCalledWith
  • toHaveReturned
  • toHaveReturnedTimes
  • toHaveReturnedWith
  • toHaveLastReturnedWith
  • toHaveNthReturnedWith
• Asymmetric matchers:
  • expect.anything
  • expect.any
  • expect.arrayContaining
  • expect.not.arrayContaining
  • expect.objectContaining
  • expect.not.objectContaining
  • expect.closeTo
  • expect.stringContaining
  • expect.not.stringContaining
  • expect.stringMatching
  • expect.not.stringMatching
• Utilities:
  • expect.addSnapshotSerializer
  • expect.assertions
  • expect.addEqualityTester
  • expect.extend
  • expect.hasAssertions

Only these functions are still not available:

• Matchers:
  • toMatchSnapshot
  • toMatchInlineSnapshot
  • toThrowErrorMatchingSnapshot
  • toThrowErrorMatchingInlineSnapshot

The tracking issue to add support for unsupported parts of the API is https://github.com/denoland/std/issues/3964.

This module is largely inspired by x/expect module by Allain Lalonde.

Add to your project Jump to heading#

deno add jsr:@std/expect

See all symbols in @std/expect on

What is expect? Jump to heading#

A Jest‑compatible fluent assertion API. Instead of standalone assert functions, you write expectations like expect(value).toBe(1) and can chain modifiers like .not, .resolves, and .rejects.

When to use @std/expect Jump to heading#

If you prefer a Jest-like API for fluent, readable assertions with rich matchers, especially when migrating from Jest.

Examples Jump to heading#

import { expect } from "@std/expect";

expect({ a: 1, b: 2 }).toMatchObject({ a: 1 });
await expect(Promise.resolve(42)).resolves.toBe(42);
expect([1, 2, 3]).toContain(2);

// Throwing and async rejections
expect(() => JSON.parse("{"))
  .toThrow(SyntaxError);
await expect(Promise.reject(new Error("boom")))
  .rejects.toThrow("boom");

// Close-to for floating point, regex matching
expect(0.1 + 0.2).toBeCloseTo(0.3, 15);
expect("hello world").toMatch(/world/);

// Using asymmetric matchers
expect({ id: 1, name: "Ada" })
  .toEqual(expect.objectContaining({ name: expect.stringMatching(/^A/) }));

Use with Deno.test Jump to heading#

import { expect } from "@std/expect";
Deno.test("basic expect", () => {
  const value = 2 + 2;
  expect(value).toBe(4);
});

For more information on testing in Deno, see the testing docs.

Tips Jump to heading#

• Use toEqual for deep data comparisons and toBe for identity.
• Prefer .resolves / .rejects helpers for promises.
• Prefer .resolves / .rejects helpers for promises.
• Use toStrictEqual for stricter deep checks (no extra props, etc.).
• Extend with custom matchers via expect.extend when needed.