q1.js

function sumf(n) {
  let sum = 0;

  for (let i = n; i > 0; i--) {
    sum += i * (n - i + 1);
  }
  
  return sum;
}

q2.js

function transform(...objs) {
  let obj = Object.assign({}, ...(objs.reverse()));
  let keys = Object.keys(obj).reverse();
  let funcKeys = keys.filter(key => typeof obj[key] === 'function');
  let promises = funcKeys.map(key => obj[key]());

  return Promise.all(promises)
    .then(keys => {
      return Promise.all(keys.map(key => {
        let value = obj[key];
        return funcKeys.includes(key) ? value() : value;
      }));
    })
    .then(result => result.filter(v => v).join(''));
}

q3.js

function letterPromises() {
  function promise(value, delay) {
    let promise = new Promise(resolve => setTimeout(() => resolve(value), delay));
    return promise.then(value => console.log(value));
  }

  return promise('a', 100)
    .then(() => Promise.all([promise('b', 100), promise('c', 100), promise('d', 100)]))
    .then(() => Promise.all([
      promise('e', 100).then(() => promise('f', 100)).then(() => promise('h', 100)),
      promise('g', 200)
    ]))
    .then(() => promise('i', 100))
    .then(() => promise('j', 100));
}