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Marcin Wosinek
Marcin Wosinek

Posted on • Originally published at how-to.dev

How to set up jest for es module

In this article, I will show a simple configuration to run jest tests for es module code.

Jest

Jest is a test framework developed by Facebook & often used in applications that use React. It's simple to set up & can be used to test any JS application.

Code

Simple code that uses export es module, src/index.js:

export function a() {
  return "function a";
}
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Test

Equally simple test, src/index.spec.js:

import { a } from "./index";

describe("function a", () => {
  it("should return expected value", () => {
    expect(a()).toEqual("function a");
  });
});
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Configuration

The default Jest configuration works fine with our example code. We need only one configuration file, .babelrc:

{
  "env": {
    "test": {
      "plugins": ["@babel/plugin-transform-modules-commonjs"]
    }
  }
}
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Dependencies

We need to install the following depenendcies:

$ npm install --save-dev jest @babel/plugin-transform-modules-commonjs
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Execution

With all that in place, we can run our test:

npm run test

> es-module-jest@1.0.0 test /home/marcin/workspace/github/es-module-jest
> jest

 PASS  src/index.spec.js
  function a
    ✓ should return expected value (2 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.329 s
Ran all test suites.
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Links

Summary

In this article, we have seen a minimalistic setup to run Jest tests for es module code.

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