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Jonas Schumacher
Jonas Schumacher

Posted on

In what order does Jest execute tests anyway? (And how to change it!)

Jest will execute different test files potentially in parallel, potentially in a different order from run to run. Per file, it will run all describe blocks first and then run tests in sequence, in the order it encountered them while executing the describe blocks.

Here's an illustration of that per file behavior, copied straight out of the doc link above:

describe('outer', () => {
  console.log('describe outer-a');

  describe('describe inner 1', () => {
    console.log('describe inner 1');
    test('test 1', () => {
      console.log('test for describe inner 1');
      expect(true).toEqual(true);
    });
  });

  console.log('describe outer-b');

  test('test 1', () => {
    console.log('test for describe outer');
    expect(true).toEqual(true);
  });

  describe('describe inner 2', () => {
    console.log('describe inner 2');
    test('test for describe inner 2', () => {
      console.log('test for describe inner 2');
      expect(false).toEqual(false);
    });
  });

  console.log('describe outer-c');
});

// describe outer-a
// describe inner 1
// describe outer-b
// describe inner 2
// describe outer-c
// test for describe inner 1
// test for describe outer
// test for describe inner 2
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If you want to run files in sequence as well, run Jest with the --runInBand command line flag. (-i does the same thing.) Doing this, the scheduler might still run them in a different order from run to run. To prevent this as well, you will need to define your own test sequencer.

To sort test files alphabetically for example, create a file with the following contents:

// testSequencer.js
const Sequencer = require('@jest/test-sequencer').default;

class CustomSequencer extends Sequencer {
  sort(tests) {
    // Test structure information
    // https://github.com/facebook/jest/blob/6b8b1404a1d9254e7d5d90a8934087a9c9899dab/packages/jest-runner/src/types.ts#L17-L21
    const copyTests = Array.from(tests);
    return copyTests.sort((testA, testB) => (testA.path > testB.path ? 1 : -1));
  }
}

module.exports = CustomSequencer;
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Then, set the following in your Jest config:

// jest.config.js
{
  "testSequencer": "path/to/testSequencer.js"
}
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Top comments (0)

Need a better mental model for async/await?

Check out this classic DEV post on the subject.

โญ๏ธ๐ŸŽ€ JavaScript Visualized: Promises & Async/Await

async await