jarle/Bun Migrate from Jest to Bun's test runner

Migrate from Jest to Bun's test runner

In many cases, Bun's test runner can run Jest test suites with no code changes. Just run bun test instead of npx jest, yarn test, etc.

npx jest # [!code --]
yarn test # [!code --]
bun test # [!code ++]

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There's often no need for code changes.

• Bun internally re-writes imports from @jest/globals to use the bun:test equivalents.
• If you're relying on Jest to inject test, expect, etc. as globals, Bun does that too.

But if you'd rather switch to the bun:test imports, you can do that too.

import { test, expect } from "@jest/globals"; // [!code --]
import { test, expect } from "bun:test"; // [!code ++]

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Since Bun v1.2.19, you can enable TypeScript support for global test functions with a single triple-slash directive. This makes migrating from Jest even easier since you only need to add the directive once in your entire project:

Add this directive to just one file in your project, such as:

• A global.d.ts file in your project root
• Your test preload.ts setup file (if using preload in bunfig.toml)
• Any single .ts file that TypeScript includes in your compilation

/// <reference types="bun-types/test-globals" />

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Once added, all test files in your project automatically get TypeScript support for Jest globals:

describe("my test suite", () => {
  test("should work", () => {
    expect(1 + 1).toBe(2);
  });

  beforeAll(() => {
    // setup code
  });

  afterEach(() => {
    // cleanup code
  });
});

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Bun implements the vast majority of Jest's matchers, but compatibility isn't 100% yet. Refer to the full compatibility table at Docs > Test runner > Writing tests.

Some notable missing features:

• expect().toHaveReturned()

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If you're using testEnvironment: "jsdom" to run your tests in a browser-like environment, you should follow the DOM testing with Bun and happy-dom guide to inject browser APIs into the global scope. This guide relies on happy-dom, which is a leaner and faster alternative to jsdom.

At the moment jsdom does not work in Bun due to its internal use of V8 APIs. Track support for it here.

[test]
preload = ["./happy-dom.ts"]

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Replace bail in your Jest config with the --bail CLI flag.

bun test --bail=3

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Replace collectCoverage with the --coverage CLI flag.

bun test --coverage

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Replace testTimeout with the --test-timeout CLI flag.

bun test --timeout 10000

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Many other flags become irrelevant or obsolete when using bun test.

• transform — Bun supports TypeScript & JSX. Other file types can be configured with Plugins.
• extensionsToTreatAsEsm
• haste — Bun uses it's own internal source maps
• watchman, watchPlugins, watchPathIgnorePatterns — use --watch to run tests in watch mode
• verbose — set logLevel: "debug" in bunfig.toml

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Settings that aren't mentioned here are not supported or have no equivalent. Please file a feature request if something important is missing.

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See also:

• Mark a test as a todo
• Docs > Test runner > Writing tests