Reduce While: Applies a reduce function as long as filter function returns true and returns the last value returned by reduceFn

example-map.js

const numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
const numbersNames = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'];

const namesOfNumbersLessThan = maxNumber => reduceWhile(
  numbers,
  number => number < maxNumber,
  (result, number, index) => result.concat(numbersNames[index])
);

console.log(namesOfNumbersLessThan(5)); // ['zero', 'one', 'two', 'three', 'four']

example-reduce.js

const numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
const numbersNames = ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine'];

const lettersOfNumbersLessThan = maxNumber => reduceWhile(
  numbers,
  number => number < maxNumber,
  (result, number, index) => result + numbersNames[index].length,
  0
);

console.log(lettersOfNumbersLessThan(5)); // 19

reduce-while.js

const defaultReduceFn = (result, element) => result.concat(element);  

export default function reduceWhile(
  elements,
  filterFn,
  reduceFn = defaultReduceFn,
  initialValue = []
) {
  let result = initialValue;

  elements.every((...args) => {
    const shouldContinue = filterFn(...args);

    if (shouldContinue) {
      result = reduceFn(result, ...args);
    }

    return shouldContinue;
  });

  return result;
}