Reducing and composing in Javascript

compose.js

'use strict';

function sortedCal(calArray) {
  return calArray.sort((a, b) => a[0] > b[0]);
}

function mergeRanges(sortedCal) {

  let start;
  let end;
  let last;

  return sortedCal.reduce((merged, currEvent) => {
    last = merged.length - 1;
    if (currEvent[0] <= merged[last][1]) {
      start = merged[last][0];
      if (currEvent[1] > merged[last][1]) {
        end = currEvent[1];
      } else {
        end = merged[last][1];
      }
      merged[last] = [start, end];
    } else {
      merged.push(currEvent);
    }
    return merged;
  }, [sortedCal[0]]);
}

function rangeHours(mergedCal) {
  return mergedCal.reduce((merged, event) => {
    merged.push([event[1] - event[0]]);
    return merged;
  }, []);
}

function compose() {
  let f2;
  const args = [...arguments];
  return function() {
    return args.reduce((accum, fx) => {
      f2 = fx.apply(null, accum);
      return [f2];
    }, [...arguments]);
  };
}

const cal = [[3, 4], [9, 12], [1, 3], [6, 8], [9, 10]];

compose(sortedCal, mergeRanges, rangeHours, f => console.log(f))(cal); // [[3, 2, 3]]