assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) function in Node.js

In this article, we will discuss the assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) function in Node.js with its syntax, parameters, and examples.

What is the assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) function?

In Node.js, an assertion error occurs when a test fails using the assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) function. It raises an AssertionError if the expected and actual values are different. It performs this by comparing the two. The comparison operator (such as ===,!==) is specified by the operator parameter, and we can customize the error message using the optional message parameter. In order to facilitate debugging, the stackStartFn option establishes the beginning point of the stack trace. This function is typically used in test cases when we want to fail a test, regardless of any other conditions forcefully. When no parameters are provided, it simply throws a generic failure.

Syntax:

It has the following syntax:

Parameters:

• Actual (Optional): The resultant value or observation made throughout the execution process. Usually, it passes when comparing the actual output of a function or expression is required.
• Expected (Optional): It is the expected value of the correct outcome. If the actual value doesn't match this expected value, the test will fail.
• Message (Optional): It is a personalized error message that appears if the assertion is unsuccessful. A default message will be generated if no message is supplied.
• Operator (Optional): It indicates the comparison operator (such as ===, ==, or!==) used to test the assertion. The '!=' is the default.
• stackStartFn (Optional): It lets us to set the starting point of the stack trace if an assertion fails. By default, it starts from the point where assert.fail() is called, but this can be customized by providing a function.

Default Behavior Without Parameters:

To indicate that the test failed without conducting a comparison, calling assert.fail() will immediately throw an AssertionError with a generic failure message if no parameters are provided.

Example:

Let us take an example to illustrate the assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) function in Node.js.

Output:

 
Example 1 - Assertion failed: The values are not equal
Example 2 - Assertion failed: 5 != 9
Example 3 - Stack trace starts from customFunction: Custom function failed
Stack trace:
 AssertionError [ERR_ASSERTION]: Custom function failed.   

Explanation:

In this example, the assert.fail() function in Node.js is used to handle assertion failures. The import of the assert module, which is necessary for testing, comes first. In Example 1, the assertion fails and a custom error message is logged. The === operator is used to check if 7 equals 8. Example 2 anticipates failure and logs the related message while carrying out a comparable check for 5 and 9 using the default != operator. Using a custom message and the !== operator, Example 3 builds a function called customFunction that determines whether 4 and 6 are equal. This assertion reports the failure message and the stack trace, which demonstrates the indication of the error if it fails.

Assertion failures are handled by the program without crashing because of the usage of try-catch blocks throughout, which facilitates efficient debugging. The code demonstrates how to handle assertion failures while simultaneously maintaining the application stable overall quite well.

Use Cases:

Several use cases of this function are as follows:

1. Explicitly Triggering Assertion Failures

In situations where a particular condition fails and is inconsistent with conventional assertions, assert.fail() can be used to indicate failure using custom logic.

2. Custom Error Messages

By giving thorough error messages that explain why an assertion failed, developers may facilitate debugging and make it simpler for others to understand the context of the failure.

3. Validating Complex Conditions

For complex validations that cannot be easily expressed with the standard assertion methods, assert.fail() allows for custom checks, enabling developers to enforce specific requirements in their code.

4. Testing Edge Cases

When normal assertions may not be sufficient, they can be used in unit tests to check for edge cases or unexpected input values. Developers may make sure that every possibility is considered by gracefully failing.

5. Controlling Assertion Behavior

Beyond basic equality checks, advanced testing is made possible through the operator argument, which allows developers to specify how the actual and expected values should be compared.

6. Stack Trace Control

It is particularly helpful in larger codebases, and when assertions are nested within functions, the stackStartFn parameter provides developers control over the stack trace and helps them identify where the assertion failed.