Improving Unit Test Efficiency: Leveraging Assert and Expect for Type Safety

In a recent client project, our development team embarked on a mission to clean up our unit tests. We noticed that our test suite had become verbose and cluttered with repetitive type checks and error handling. We knew there had to be a more efficient way to ensure type safety without sacrificing the clarity and conciseness of our tests. While pursuing better testing practices, we realized the power of assert statements.
By replacing explicit error-throwing with concise one-liners, we significantly improved the readability and maintainability of our unit tests.

Additionally, the assert statements provided precise and descriptive failure messages, enabling us to pinpoint and address potential issues quickly.


Consider this example:

describe('Testing the fighters array', () => {
it('should return the correct data for an existing fighter', () => {
const foo = fighters.find((foo) => ===;

if (foo === undefined) {
throw new TypeError('Fighter with specified ID does not exist');


The test checks if the fighter with the specified ID exists and then proceeds to use expect to validate the foo.value against expectedData.

Throwing errors within tests has always felt like a bad practice unless you are explicitly testing error handling. Instead, it’s best to have errors bubbling up from a service or repository layer.

Understanding Assert and Expect Statements

JavaScript testing frameworks offer developers access to two essential tools for making assertions: assert and expect statements. While they share similarities in their purpose, there are strategic ways to harness their power effectively.

The primary distinction between assert and expect lies in their syntax and expressiveness. Assert statements employ concise one-liners to perform equality checks and truthiness assertions. Whereas expect statements provide a more expressive syntax, enabling developers to craft descriptive and complex test cases.

When a test fails, the error messages provided by assert and expect differ. Assert statements provide succinct messages when a test fails, typically showing the expected and actual values that caused the failure.

On the other hand, expect statements offer more comprehensive and informative failure messages. They not only display the expected and actual values but also provide contextual details, such as property names or nested structures, enabling developers to pinpoint the specific cause of the test failure quickly.

This enhanced visibility into the test results empowers developers to debug and resolve issues efficiently, streamlining the development process.

When to Use Assert vs. Expect

Assert is excellent for making simple equality checks, effectively replacing traditional type checking. It allows us to ensure that expected outcomes meet specific conditions without cluttering our tests with cumbersome type checks.

Expect is ideal for testing complex conditions on nested structures or deeply nested objects. Its expressive syntax enables us to write more detailed and readable test cases.

Leveraging Assert for Type Safety

While assert and expect can be used separately, they complement each other well, allowing our tests to focus on the logic we genuinely care about testing.

describe('Testing the fighters array', () => {
it('should find the fighter with the specified ID', () => {
const foo = fighters.find((foo) => ===;

assert(foo !== undefined);

By replacing the if statement with an assert statement in the previous example, we immediately eliminate at least two lines of code and improve the type inference for foo.

Benefits of Using Assert and Expect Together

  1. Improved readability. Combining assert and expect allows developers to write more expressive and human-readable tests, making it easier to understand the intent of the tests.
  2. Enhanced type safety. Leveraging assert and expect enables developers to ensure type safety without requiring lengthy type checks or custom error throwing.
  3. Clear failure messages. Expect provides descriptive failure messages, simplifying issue diagnosis and resolution.

Leveraging Assert and Expect

While these examples are simple, the more complicated your test data and types are, the greater chance you may want to use assert statements strategically.

Unit tests are essential for maintaining code quality and catching bugs early in the development process. By using assert and expect statements together, developers can create concise, expressive, and type-safe tests. This combination strengthens the overall testing process, making it easier to write and maintain tests while ensuring code reliability.

Happy testing!


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