Mastering JavaScript Functional Programming - Second Edition

Testing the solution manually

We can run a simple test. Let's write a squeak() function that will, appropriately, squeak when called! The code is simple:

const squeak = a => console.log(a, " squeak!!");squeak("original"); // "original squeak!!"squeak("original"); // "original squeak!!"squeak("original"); // "original squeak!!"

If we apply once() to it, we get a new function that will squeak only once. See the highlighted line in the following code:

const squeakOnce = once(squeak);squeakOnce("only once"); // "only once squeak!!"squeakOnce("only once"); // no outputsqueakOnce("only once"); // no output

Check out the results at CodePen or see Figure 2.2:

Figure 2.2: Testing our once() higher-order function

The previous steps showed us how we ...