Simple Callback to Promise Conversion

Question

I have a simple callback example (more like pseudo-code) where values are summed up or multiplied based on the first function parameter.
This is not like a real-life example. Imagine that the task/calculation inside of functions takes some time (like for e.g. calculating the number of stars in the universe). But if we put this aside, the pattern/template with callback looks like this: 

let sum = function(a, b) {
  return a + b;
}

let mul = function(a, b) {
    return a * b;
}

let dne = function(a, b) {
    return 'it does not exist';
}

let doCalculation = function(route) {
  switch(route) {
      case 'sum':
        return sum;
      case 'mul': 
        return mul
      default: 
        return dne
  }
}

console.log(doCalculation('sum')(3, 4)) //7

Could this be refactored with Promises in a similar way and if it could be done, could it be done simpler.

Answer 1

You can just convert the functions into async functions/promises.
Some good documentation about that can be found here .

But I guess you also want to simulate some delay. So I added additional function for that (it might help with understanding).

const sleep = (delay) => new Promise((resolve) => setTimeout(resolve, delay))

const sumWithDelay = async (a, b) => {
  console.log("Delay started")
  await sleep(3000)
  console.log("Delay ended")
  return a + b;
}

const sum = async (a, b) => {
  return a + b;
}

const mul = async (a, b) => {
  return a * b;
}

let dne = async function() {
  return 'it does not exist';
};

let doCalculation = function(route) {
  switch (route) {
    case 'sum':
      return sum;
    case 'sumWithDelay':
      return sumWithDelay;
    case 'mul':
      return mul;
    default:
      return dne;
  }
};

doCalculation('sumWithDelay')(2, 2).then(res => console.log(res)); //4
doCalculation('sum')(3, 4).then(res => console.log(res)); // 7
doCalculation('mul')(3, 4).then(res => console.log(res)); // 12
doCalculation('dne')(3, 4).then(res => console.log(res)); // it does not exist

The output will be:
Delay started
7
12
it does not exist
Delay ended
4

You can see that the "sum with delay" has been executed last (after 3 sec).