A little Javascript challenge

sequence_fun.js

/**
 * Given an array of "steps", execute them while accounting for delay
 * See: https://medium.com/@kitze/js-coding-challenge-1-test-your-skills-63c2af5446d0#.n2wq6jyqu
 * @param {function|int[]} steps
 */
const sequence = (steps) => steps.reduce(
  // Reduce the steps, keeping track of an accumulated timeout
  (timeout, step) => typeof step === 'function'
    // Yes: invoke it with the currently accumulated timeout
    ? setTimeout(step, timeout) && timeout
    // No: it is an integer. Increase accumulated timeout
    : timeout + step
  , 0);

JSBin link here: http://jsbin.com/maqopig/edit?js,console