Reduce your fears about .reduce() - twentySomethingsLongFullNames-reduce.js

twentySomethingsLongFullNames-reduce.js

const isInTwenties = user => user.age >= 20 && user.age < 30;
const makeFullName = user => `${user.firstName} ${user.lastName}`;
const isAtLeastTenChars = fullName => fullName.length >= 10;

const twentySomethingsLongFullNames = users.reduce(
  //  The first arg that .reduce takes is the reducer function
  //  It has the signature: 
  //  - `accumulator` - this is what you build up as you move through the array
  //  - `user`        - this is the reference to the item in the array we are currently on
  //  - `index`       - not used here, but you can get the index of the current place in the array you are
  //  - `array`       - not used here, but this refers to the original array if you need a ref to it
  (accumulator, user) => {
      const fullName = makeFullName(user);
      if (isInTwenties(user) && isAtLeastTenChars(fullName)) accumulator.push(fullName);
      return accumulator
  },
  //  The second argument (optional) is the initial value
  //  This will be the value of `accumulator` in the first iteration of `.reduce`
  []
);