Async/await transpiled by hand to Promises

asyncawaitvspromises.js

function add(a, b) {
  return new Promise((resolve, reject) => {
    resolve(a + b);
  });
} // return promise

function multiply(x, y) {
  return new Promise((resolve, reject) => {
    resolve(x * y);
  });
} // return promise

async function mainAsync() {
  let sum, product;
  sum = await add(2, 3); // 5
  product = await multiply(2, 2); // 4
  console.log('async', 'sum + product', sum + product);
  return 'ok';
}

function mainPromise() {
  const mainPromise = new Promise(resolve => {
    let sum, product; // 'hoisted' variables
    
    const block1 = () => {
      return add(2, 3);
    };
    const block2 = resolvedValue => {
      sum = resolvedValue; // 5
      return multiply(2, 2);
    };
    const block3 = resolvedValue => {
      product = resolvedValue; // 4
      console.log('promise', 'sum + product', sum + product);
      return 'ok';
    };

    const returnPromiseFromChain = block1() // Promise => 'ok'
      .then(block2)
      .then(block3);

    resolve(returnPromiseFromChain);
  });

  return mainPromise;
}

mainAsync()
  .then(returnValue => {
    console.log('async returned:', returnValue);
  })
  .catch(err => {
    console.error(err);
  });

mainPromise()
  .then(returnValue => {
    console.log('promise returned:', returnValue); // 'ok'
  })
  .catch(err => {
    console.error(err);
  });

// Output:
// async sum + product 9
// promise sum + product 9
// async returned: ok
// promise returned: ok