Migrating from Jest

Most Jest suites translate to deno test without rewriting test logic: the node:test module provides the same describe/it structure, and the standard library ships an expect with the matchers you already use. What changes is a handful of imports and how the runner is configured.

The same test, in Deno

A typical Jest test needs only new imports:

import { beforeEach, describe, it } from "node:test";
import { expect, fn } from "jsr:@std/expect";

describe("shopping cart", () => {
  let cart: string[];

  beforeEach(() => {
    cart = [];
  });

  it("starts empty", () => {
    expect(cart).toHaveLength(0);
  });

  it("notifies a listener", () => {
    const onAdd = fn();
    cart.push("apple");
    onAdd(cart.length);
    expect(onAdd).toHaveBeenCalledWith(1);
  });
});

$ deno test cart.test.ts
ok | 1 passed (2 steps) | 0 failed (13ms)

Unlike Jest, nothing is injected as a global: describe, it, expect, and the hooks are explicit imports, so each file states what it uses. Tests written against node:test also keep working in Node.js itself.

What maps to what

Translate the configuration

Jest configuration mostly disappears: TypeScript, JSX, and ES modules work without transforms, so ts-jest, babel-jest, and transform entries have no equivalent to migrate. File selection moves into deno.json:

{
  "test": {
    "include": ["src/**/*.test.ts"],
    "exclude": ["src/fixtures/"]
  }
}

Module mocks

There is no equivalent of jest.mock("./module"), because module records are immutable in Deno. Tests that rely on it migrate to one of:

• dependency injection: pass the collaborator in, and hand the test a spy or stub,
• stubbing the object the module exposes, with stub from @std/testing/mock.

This is usually the only part of a migration that requires touching the code under test.

Keep npm packages where it helps

Test helpers from npm keep working through npm: specifiers, for example npm:@testing-library/dom for DOM assertions (see Use Testing Library with Deno). The test runner itself is the part you migrate.

For runner flags, reporters, and coverage, see the deno test reference.