Mastering JavaScript Functional Programming - Second Edition

Logically negating a function

Let's consider the filter() method from Chapter 5, Programming Declaratively – A Better Style. Given a predicate, we can filter the array to only include those elements for which the predicate is true. But how would you do a reverse filter and exclude the elements for which the predicate is true?

The first solution should be pretty obvious: rework the predicate so that it returns the opposite of whatever it originally returned. In Chapter 5, Programming Declaratively – A Better Style, we looked at the following example:

const delinquent = serviceResult.accountsData.filter(v => v.balance < 0);

So, we could just write it the other way round, in either of these two equivalent fashions. Note the different ways of ...