Enhancing Error Assertions for Better Test Coverage

Oct 13, 2025 by pynkocean.com 88 views

Enhancing Error Assertions for Better Test Coverage

When working with custom error types, it's essential to have robust assertion methods to ensure that our tests accurately reflect the expected behavior. The toThrowWithMessage method is a powerful tool for asserting error messages, but it can be limited when dealing with custom error types that have additional fields. In this article, we'll explore how to enhance toThrowWithMessage to support asserting extra fields besides just the error class type and message.

The Problem with `toThrowWithMessage`

The toThrowWithMessage method is designed to assert that a function throws an error with a specific message. However, when dealing with custom error types that have additional fields, this method can be insufficient. For example, consider the following custom error type:

class ExecutionError extends Error {
  exitCode: number;
  failedCommand: string;

  constructor(errorMessage: string, exitCode: number, failedCommand: string) {
    super(errorMessage);
    this.exitCode = exitCode;
    this.failedCommand = failedCommand;
  }
}

In this example, we have a custom error type ExecutionError that extends the built-in Error class. This error type has two additional fields: exitCode and failedCommand. When writing tests for this error type, we might want to assert that the failedCommand field is set correctly. However, using the toThrowWithMessage method alone is not sufficient, as it only checks the error message.

The Limitations of `toStrictEqual`

Another approach to asserting the failedCommand field is to use the toStrictEqual method. However, this method is not designed to work with custom error types that have additional fields. When we try to use toStrictEqual with our custom error type, we get the following error:

expect(new ExecutionError('Error message', 1, 'Failed command')).toStrictEqual({
  exitCode: 1,
  failedCommand: 'Failed command',
  message: 'Error message',
});

This error occurs because the toStrictEqual method checks for strict equality between the expected object and the actual object. Since our custom error type has a message property that is not present in the expected object, the assertion fails.

Enhancing `toThrowWithMessage` for Custom Error Types

To overcome the limitations of toThrowWithMessage and toStrictEqual, we can create a custom assertion method that checks for the presence of additional fields in our custom error type. Here's an example of how we can do this:

expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toThrowWithMessage(ExecutionError, 'Error message');
expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toHaveProperty('exitCode', 1);
expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toHaveProperty('failedCommand', 'Failed command');

In this example, we use the toThrowWithMessage method to assert that the error message is correct, and then use the toHaveProperty method to assert that the exitCode and failedCommand fields are set correctly.

Creating a Custom Assertion Method

To make our custom assertion method more reusable, we can create a function that takes the expected error type and the expected properties as arguments. Here's an example of how we can do this:

function toThrowWithProperties(errorType, message, ...properties) {
  return (fn) => {
    expect(() => fn()).toThrowWithMessage(errorType, message);
    properties.forEach((property) => {
      expect(() => fn()).toHaveProperty(property);
    });
  };
}

With this custom assertion method, we can write more concise and readable tests for our custom error types. For example:

expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toThrowWithProperties(ExecutionError, 'Error message', 'exitCode', 'failedCommand');

Conclusion

Frequently Asked Questions about Custom Error Assertions

In our previous article, we explored how to enhance the toThrowWithMessage method to support asserting extra fields besides just the error class type and message. In this Q&A article, we'll answer some of the most frequently asked questions about custom error assertions.

Q: Why do I need to enhance `toThrowWithMessage`?

A: The toThrowWithMessage method is a powerful tool for asserting error messages, but it can be limited when dealing with custom error types that have additional fields. By enhancing toThrowWithMessage, you can ensure that your tests accurately reflect the expected behavior of your custom error types.

Q: How do I create a custom error type?

A: To create a custom error type, you can extend the built-in Error class. For example:

class ExecutionError extends Error {
  exitCode: number;
  failedCommand: string;

  constructor(errorMessage: string, exitCode: number, failedCommand: string) {
    super(errorMessage);
    this.exitCode = exitCode;
    this.failedCommand = failedCommand;
  }
}

Q: How do I use the `toThrowWithProperties` method?

A: To use the toThrowWithProperties method, you can pass the expected error type, the expected message, and the expected properties as arguments. For example:

expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toThrowWithProperties(ExecutionError, 'Error message', 'exitCode', 'failedCommand');

Q: Can I use the `toThrowWithProperties` method with other assertion libraries?

A: Yes, you can use the toThrowWithProperties method with other assertion libraries, such as Jest or Mocha. The method is designed to be flexible and can be used with any assertion library that supports the toThrowWithMessage method.

Q: How do I handle nested properties in my custom error type?

A: To handle nested properties in your custom error type, you can use the toHaveProperty method with a nested path. For example:

expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toHaveProperty('exitCode', 1);
expect(() => {
  throw new ExecutionError('Error message', 1, 'Failed command');
}).toHaveProperty('failedCommand.failedSubCommand', 'Failed sub-command');

Q: Can I use the `toThrowWithProperties` method with async functions?

A: Yes, you can use the toThrowWithProperties method with async functions. The method is designed to work with both sync and async functions.

Q: How do I customize the `toThrowWithProperties` method to fit my needs?

A: To customize the toThrowWithProperties method, you can create a custom implementation that meets your specific requirements. For example, you can add additional properties or modify the existing behavior to fit your needs.

Conclusion

In this Q&A article, we answered some of the most frequently asked questions about custom error assertions. We demonstrated how to create a custom error type, use the toThrowWithProperties method, and handle nested properties. By using the toThrowWithProperties method, you can ensure that your tests accurately reflect the expected behavior of your custom error types.