Test a Function Throwing Error

Learn to create a test that captures when a function throws.


When a function throws as part of its logic, there should be a test case that captures that.

The function

The following function accepts a string text and a number n and returns the first n characters of the string.

function getFirstNChars(text, n) {
  return text.slice(0, n);

The implementation uses the slice method out of the string.prototype (click here for details). It slices the passed in text from index 0 to n.


Since this lesson focuses on testing the error cases (below), first let’s try to implement this test case:

“The firsNChars function should return the first n characters from a string!”

Use src/first-n-chars.spec.js in the code playground below.

This first spec should test the intended use case, also known as the green path.

If stuck, see the hint below the code widget.

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