Now that we have written and refactored a few tests, it’s time to consider edge cases.

An edge case is a problem or situation that occurs only at an extreme (maximum or minimum) operating parameter — you can think of these as special cases that you need to account for.

Take our Phrase.initials() example from earlier. What happens if we pass .initials() a number instead of a string? We could write a test and then implementation code that responds to this behavior by raising an error message that instructs you to only use strings.

The test for throwing an error when a number is passed to Phrase.initials() would look like this:

it('raises an error if the parameter passed in is not a string', () => {
  // Setup
  const nameInput = 14;   
  // Exercise
  const exercise = () => { Phrase.initials(nameInput) };
  // Verification
  assert.throws(exercise, /only use string/);        
})

After getting our intended error message we would write just enough implementation code to pass the test. Which in this case would be adding the following to Phrase.initials():

if (typeof inputName !== "string") {
  throw new Error("ERROR: only use string");
}

The complete implementation code would now be back in the green and look like this:

const Phrase = {
  initials(inputName) {
    if (typeof inputName !== "string") {
      throw new Error("ERROR: only use string");
    }
    const initials = [];
    const words = inputName.split(" ");

    words.forEach((word) => {
      initials.push(word.charAt(0));
    });

   return initials.join("");
  }
}

This is an example of creating a test for edge cases.