Solution to https://medium.com/@kitze/js-coding-challenge-1-test-your-skills-63c2af5446d0

sequence-four-solutions.js

/**
 * This solves the problem using Lodash's _.reduceRight,
 * it runs backwards on the array, and makes each function/int to trigger the next ones
*/
function sequence(list = []){
  _.reduceRight(list, (next, item) => () => _.isFunction(item) ? next(item()): setTimeout(next, item), () => {})();
}

/**
 * This solves it without using iteration, but tail recursion,
 * it runs each item and then calls recursively on the rest.
 * It brilliantly uses the setTimeout to either run the function, or delay with the int
*/ 
function sequence([item = null, ...rest]){
  item && setTimeout(() => sequence(rest), _.isFunction(item) ? (item() , 0) : item);
}

/**
 * Same as the previous solution (recursion, rest params & brilliance),
 * but without running the functions themselves in setTimeout
*/ 
function sequence([item = null, ...rest]){
  item && (_.isFunction(item) ? sequence(rest, item()) : setTimeout(() => sequence(rest), item));
}

/**
 * This solves it using the old fashioned map/reduce,
 * where each item turns to a Promise, and the the promises are run sequentially.
*/ 
function sequence(list = []) {
  list
    .map(item => () =>  new Promise(resolve => _.isFunction(item) ? resolve(item()) : setTimeout(resolve, item)))
    .reduce((prev, next) => prev.then(next), Promise.resolve());
}