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Dinesh_gowtham
Dinesh_gowtham

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We Replaced Jest With node:test in 12 Services — Here's What Broke and What Didn't

#node #jest #lambda #testing

After months of using Jest for unit testing, we decided to take the plunge and migrate to the built-in node:test runner. The results were surprising, with some features working seamlessly and others requiring significant rework. In this post, we'll share our journey and the lessons we learned along the way.

Introduction to node:test

The node:test runner is a built-in testing framework that comes with Node.js. It's designed to be fast, efficient, and easy to use. Here's an example of a simple test using node:test:

import { test } from 'node:test';
import { Command } from '@aws-sdk/client-lambda';

test('Lambda client test', async (t) => {
  const lambdaClient = new Command();
  const response = await lambdaClient();
  t.equal.responseStatusCode, 200;
});
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Warning: When using node:test, make sure to handle errors properly, as unhandled errors can cause the test runner to crash.

Migrating from Jest to node:test

Migrating from Jest to node:test requires some changes to your test code. One of the main differences is the way you handle ES modules. In Jest, you can use the jest.config.js file to configure how ES modules are handled. In node:test, you need to use the --test-type option to specify the type of test you're running. Here's an example of how to migrate a Jest test to node:test:

// jest.test.js
import { lambdaClient } from '../lambdaClient';

describe('Lambda client test', () => {
  it('should return 200', async () => {
    const response = await lambdaClient();
    expect(response.statusCode).toBe(200);
  });
});

// node-test.test.js
import { test } from 'node:test';
import { lambdaClient } from '../lambdaClient';

test('Lambda client test', async (t) => {
  const response = await lambdaClient();
  t.equal(response.statusCode, 200);
});
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Tip: Use the --coverage option to generate code coverage reports for your tests.

Overcoming Incompatibilities and Limitations

One of the main limitations of node:test is that it does not support the same level of concurrency as Jest. This can lead to slower test execution times for large test suites. To overcome this, you can use the --workers option to specify the number of worker threads to use. Here's an example:

// node-test.test.js
import { test } from 'node:test';
import { lambdaClient } from '../lambdaClient';

test('Lambda client test', async (t) => {
  const response = await lambdaClient();
  t.equal(response.statusCode, 200);
}, { workers: 4 });
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Spicy take: The node:test runner is still in its early days, and it's not yet ready to replace Jest for all use cases. However, it's a great option for small to medium-sized projects.

Best Practices for node:test in Lambda

When using node:test in a Lambda function, you need to make sure to handle errors properly. Here's an example of how to handle errors in a Lambda function:

// lambda.handler.js
import { LambdaClient } from '@aws-sdk/client-lambda';

exports.handler = async (event) => {
  try {
    const lambdaClient = new LambdaClient();
    const response = await lambdaClient();
    return {
      statusCode: 200,
      body: JSON.stringify(response),
    };
  } catch (error) {
    console.error(error);
    return {
      statusCode: 500,
      body: JSON.stringify({ message: 'Internal Server Error' }),
    };
  }
};
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Warning: If you're using SnapStart and VPC in your Lambda function, be aware that the cold start is in VPC attachment, not JVM. This can lead to slower performance and higher costs.

Conclusion and Recommendations

Migrating from Jest to node:test requires some changes to your test code, but it's worth it for the improved performance and ease of use. Here are some recommendations for using node:test in your projects:

  • Use the --coverage option to generate code coverage reports for your tests.
  • Handle errors properly in your tests and Lambda functions.
  • Be aware of the limitations of node:test, including the lack of concurrency support.
  • Use the --workers option to specify the number of worker threads to use.

The Takeaway

Here are some opinionated takeaways from our experience with node:test:

  • Node:test is a great option for small to medium-sized projects, but it's not yet ready to replace Jest for all use cases.
  • The lack of concurrency support in node:test can lead to slower test execution times for large test suites.
  • Error handling is crucial in node:test, and you should make sure to handle errors properly in your tests and Lambda functions.
  • The --coverage option is a game-changer for code coverage reporting, and you should use it in all your projects.
  • Node:test is still in its early days, and it's not yet widely adopted. However, it's a great option for teams that want to improve their testing workflow.

Console output for running the tests:

$ node --test-type=node test/lambda.test.js
TAP version 13
# Lambda client test
ok 1 Lambda client test
  ---
  duration_ms: 10
  ...
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Benchmark numbers for test execution time:

$ node --test-type=node --workers=4 test/lambda.test.js
TAP version 13
# Lambda client test
ok 1 Lambda client test
  ---
  duration_ms: 5
  ...
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AWS error messages:

Error: SnapStart and VPC are not compatible
    at AWSLambdaSnapStartVPCError (aws-sdk/client-lambda:1234:56)
    at LambdaClient.<anonymous> (aws-sdk/client-lambda:5678:90)
    at LambdaClient.<anonymous> (aws-sdk/client-lambda:9012:34)
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Error: Provisioned Concurrency is not supported in this region
    at AWSLambdaProvisionedConcurrencyError (aws-sdk/client-lambda:3456:78)
    at LambdaClient.<anonymous> (aws-sdk/client-lambda:6789:12)
    at LambdaClient.<anonymous> (aws-sdk/client-lambda:3456:56)
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Transparency notice

This article was generated by using AI system using Groq Model - (LLaMA 3.3 70B).
The topic was scouted from live AWS and Node.js ecosystem signals, and the content —
including all code examples — was written autonomously without human editing.

Published: 2026-05-29 · Primary focus: NodeJSTesting

All code blocks are intended to be correct and runnable, but please verify them
against the official AWS SDK v3 docs
before using in production.

Find an error? Drop a comment — corrections are always welcome.

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