How to Unit Test a JavaScript String Capitalization Function

A simple JavaScript function called capitalize takes a string input and converts its first letter to uppercase while leaving the rest unchanged. The function includes a guard clause that returns the original value if the input is falsy, such as an empty string. Unit tests for this function are written using a standard testing framework, covering four key scenarios: basic capitalization, already-capitalized input, single characters, and empty strings. Each test uses an expect-toBe assertion to verify that the function returns the correct output for a given input. This example illustrates how even small utility functions benefit from thorough unit testing to confirm expected behavior across edge cases.