Mastering JavaScript Functional Programming - Second Edition

Generalizing for all reducers

To be able to work with all kinds of reducers and produce whatever kind of result they build, we'll need to make a small change. The idea is simple: let's modify our makeReducer() function so that it will accept a final reducer and a starting value for the accumulator:

const makeReducer2 = (arr, fns, reducer = addToArray, initial = []) =>  arr.reduce(compose(...fns)(reducer), initial);const a3 = makeReducer2(myArray, [  testOddR,  duplicateR,  testUnderFiftyR,  addThreeR,]);/*[ 21, 25 ]*/

To make this function more usable, we specified our array-building function (and [] as a starting accumulator value) so that if you skip those two parameters, you'll get a reducer that produces an array. Now, let's look at the other ...