Writing Several Unit Tests Together (Multiple Test Cases)

In this lesson you'll learn how to run several test cases together, so you can efficiently confirm that a function is correct. This is for people searching "JavaScript how to write test cases" or "JavaScript unit testing for beginners."

Testing a single function against multiple input patterns all together is called unit testing. Preparing a variety of cases in advance — "normal values," "boundary values," "special values" — helps you catch bugs sooner. Managing your test cases together in an array also makes it easy to add more cases later.

The sample code gathers several combinations of input and expected value into an array called testCases, and uses .forEach() to compare the result of the add() function against the expected value, one case at a time. It counts how many tests passed in a variable called passed, and finally displays a summary result like "X out of Y passed."

A common beginner stumbling block is testing only normal values and forgetting to test "boundary values" or "special values" like 0, negative numbers, or an empty array. The idea of boundary-value testing is an important perspective for deliberately checking these "special situations," since many real bugs actually occur right at these boundary conditions.

In real development, it's standard practice to run tests like this automatically every time you modify code, continually confirming that existing features haven't broken. Rather than relying on manual checking, it's worth building this habit of preserving quality through automated tests early on.