Improve this page

"jest" task

This task invokes the Jest test framework for unit testing.

When to use it

We recommend Jest for several reasons:

  • All-in-one: Unlike frameworks such as mocha that require many components to be hooked together, Jest provides a single integrated solution for your: test runner, assertion library, mock/spy API, instrumentation, code coverage, and reporting. Jest also has first class support for React.

  • Interactive: Jest supports a “watch mode” for automatically re-running tests whenever a file is saved, plus a snapshot testing that can automatically update assertions when a contract changes.

  • Isolated runtime: Jest runs web tests in a Node.js environment rather than launching a web browser, and leverages the Node.js VM feature to prevent tests from leaking state.

That said, if for some reason you need to run tests in some other runtime such as an Android client or real web browser, Jest may not be the best bet.

package.json dependencies

Heft has direct dependencies on the Jest packages that it needs, so you don’t need to add Jest to your project’s package.json file.

Your project should get its typings from @types/heft-jest instead of @types/jest:

$ rush add --package @types/heft-jest --exact --dev

…and then reference heft-jest in your tsconfig.json file, like this example:

  "extends": "./node_modules/@rushstack/heft-node-rig/profiles/default/tsconfig-base.json",
  "compilerOptions": {
    "types": [
      "heft-jest", // <---- ADD THIS

Config files

There isn’t a Heft-specific file for this task. Heft looks for Jest’s config file in the standard path config/jest.config.json. Although Jest itself supports other config file names and even embedding settings in your package.json file, Heft only supports the name jest.config.json. Using one standard filename makes it easy to search these files, perform bulk edits, and copy configuration recipes between projects.

Generally your Jest configuration should simply extend Heft’s standard preset:

<project folder>/jest.config.json

  "preset": "./node_modules/@rushstack/heft/includes/jest-shared.config.json"

Note: If you find yourself frequently adding lots of custom settings to this file, please create a GitHub issue and tell us about it. Our aim is to provide a configuration that minimizes the need for project-specific customizations.

Differences from ts-jest

Internally, Jest supports TypeScript compilation via plugins called transforms, which are modeled as a synchronous function that receives a single .ts file as input, and returns a .js file and .map file as its output. The official babel-jest transform actually does compile one file at a time, but that approach cannot support language features such as const enum that require analyzing imported types. The ts-jest transform solves this problem by performing a full compiler analysis and reusing it each time the transform is invoked, but this won’t support other build steps such as preprocessors. Both babel-jest and ts-jest also impose a significant performance cost, by invoking the compiler a second time when running tests.

Heft takes a different approach of performing a conventional build and then invoking Jest on the output. If your build targets a browser runtime, you’ll need to use the emitFolderNameForTests setting to emit CommonJS outputs in a secondary folder. (Emitting extra files is still significantly faster than invoking the compiler twice.) Heft’s jest-build-transform.js does not compile anything itself, but rather returns the output of the full pipeline.

Some helpful examples of Jest techniques can be found in the heft-node-jest-tutorial project folder.

Important differences when using Jest with Heft:

  • Invoke Jest using the heft command line. Invoking the jest command line will successfully run tests, but it will not compile anything.

  • Do not add ts-jest or babel-jest as a dependency for your project.

  • Instead of import { mocked } from "ts-jest/utils";, use the global mocked() function that is provided by @types/heft-jest. Besides this difference, the API documentation from ts-jest is still applicable to Heft’s implementation.

  • The ts-jest transform will magically “hoist” calls to jest.mock();. Heft does not consider this a best practice. Instead, use the @rushstack/hoist-jest-mock lint rule to remind developers to manually hoist their calls. It is enabled by default with @rushstack/eslint-config.

Debugging Jest tests

To debug your Jest tests, it’s recommended create a VS Code launch.json file like this:


  "version": "0.2.0",
  "configurations": [
      "type": "node",
      "request": "launch",
      "name": "Debug Jest tests",
      "program": "${workspaceFolder}/node_modules/@rushstack/heft/lib/start.js",
      "cwd": "${workspaceFolder}",
      "args": ["--debug", "test", "--clean"],
      "console": "integratedTerminal",
      "sourceMaps": true

This launches the full Heft toolchain in your debugger. The --debug switch prevents Jest from being spawned as a separate process. The --clean flag is optional, but fixes an issue where in rare situations Jest’s “haste-map” may become corrupted by an aborted run.

To restrict the debugger to run one specific test, you can add the --test-name-pattern parameter. (See here for command-line documentation.) Another option is to use Jest’s test.only() API.

See also